Smtp error code 500

What are SMTP codes and how to troubleshoot them

Simple Mail Transfer Protocol (SMTP) is the standard communication protocol for sending emails over the Internet.

Getting started with SMTP for your transactional emails is quick and easy. You simply provide your server name, port, username and password. The SMTP server then sends emails on your behalf and takes care of email deliverability.

However, when an SMTP error prevents your emails from getting delivered, it can be difficult to understand its cryptic error codes in your delivery reports. Read on to learn how to read common SMTP response codes and how you can troubleshoot delivery errors.

What are SMTP codes

SMTP response codes are updates sent by an SMTP server during the mail delivery process. Using a 3-digit format, they are sent in response to commands issued by an SMTP client. The following diagram shows the relationship between the SMTP client and server in the mail delivery process.

How to read SMTP codes

The SMTP specification defined basic status codes that were published in 1982. Because the codes were originally designed to report on the outcome of an SMTP command, their lack of detail made them unsuitable for delivery reports.

To compound the problem, some servers would assign the same error code to different delivery failures. Others would assign a code that only explained its meaning in the text description. Inconsistent application of the codes meant it was difficult to resolve mail sending issues!

Enter the enhanced status codes. They extend the basic status codes by providing more detail about the cause of the email delivery failure. Similarly, they also consist of 3-digit numbers separated by decimals. Let’s take a look at how to read them next.

Basic status codes

The first digit of a basic status code tells the sending server whether the response is good, bad or incomplete:

2: Positive Completion Reply. The requested action has been successfully completed.

3: Positive Intermediate Reply. The command is accepted but more information is needed before proceeding.

4: Transient Negative Completion Reply. The command was not accepted and no action was taken. This error is seen as temporary (soft bounce) and the sender can try again later.

5: Permanent Negative Completion Reply. The command was not accepted and no action was taken. This is a permanent error (hard bounce) and the sender should not repeat the command.

The second digit puts the SMTP response into a specific category:

: Syntax. Responses about command or parameters.

1: Information. Responses to requests for more information.

2: Connections. Responses about the transmission channel.

5: Mail system. Responses about the status of the receiving mail server.

The third digit goes into more detail within the response category.

For example, if you telnet to MailerSend’s SMTP server and introduce yourself with the HELO command, you will see the following exchange:

The 250 response code means that it is a positive completion reply about the status of the mail system, i.e. the SMTP server has acknowledged your IP address and is waiting for your next command.

Enhanced status codes

Like the basic status codes, the first digit defines the class to which the code belongs:

2: Success! The requested mail action is okay and completed.

4: Persistent Transient Failure. Temporary conditions will cause the message to be delayed or abandoned.

5: Permanent Failure. The message in its current form cannot be delivered.

The second digit identifies the subject of the SMTP reply code:

: Other or Undefined Status

1: Addressing Status

2: Mailbox Status

3: Mail System Status

4: Network and Routing Status

5: Mail Delivery Protocol Status

6: Message Content or Media Status

7: Security or Policy Status

Finally, the third digit provides more detail and it is dependent on the class and the subject.

For example, if you specify the recipient of the email with the RCPT TO command, the SMTP server may respond with 250 2.1.5 Recipient OK. This code means that not only is your command successful, the address of the recipient at the server has been accepted as well.

Common SMTP codes and how to troubleshoot them

The current SMTP standard defines the response codes but they are applied differently between SMTP servers. Not every mail server is configured in the same way and thus there will be differences in how mail delivery failures are interpreted.

General SMTP status codes

SMTP codes that start with 2 and 3 are general status messages that are sent in reply to requests from the sending server. They do not indicate errors in the mail delivery but are used as prompts in the exchange between the mail servers.

214 Help message. A response to the HELP command that usually includes a link or URL to the FAQ page.

220 SMTP Service ready. The receiving server is ready for the next command.

221 Service closing transmission channel. The receiving server is closing the SMTP connection.

235 2.7.0 Authentication succeeded: The sending server’s authentication is successful.

250 Requested mail action okay, completed. Success! The email was delivered.

251 User not local; will forward to . The receiving server doesn’t recognize the recipient but it will forward it to another email address.

252 Cannot VRFY user, but will accept message and attempt delivery. The receiving server doesn’t recognize the recipient but it will try to deliver the email anyway.

354 Start mail input. The email “header” has been received, the server is now waiting for the “body” of the message.

Temporary SMTP error codes

SMTP 400 codes are transient errors and are usually related to issues at the receiving mail server. Messages are returned as a soft bounce and the sender may try again after, for example, reducing the size of the attachments in the email.

421 Service not available, closing transmission channel. The receiving server or sending server is not reachable but another mail delivery will be attempted. If you are using a remote server like MailerSend’s SMTP relay to send emails, test that you can connect to it successfully. Otherwise, you may wish to check the receiving server’s availability.

450 Requested mail action not taken: mailbox unavailable. This error could mean that the recipient does not exist, the mailbox does not have permission to receive the email, or the message was rejected due to a blocklist or filter.

To fix this, you should confirm the recipient’s email address, see if your IP address is on a blocklist, test your email for spam-like content, or try sending an email without an attachment.

451 Requested action aborted: error in processing. The receiving server is unable to process the message due to email authentication rules. If Sender Policy Framework (SPF) is being used, for example, you should confirm that a DNS lookup can be made for your sending domain and that the domain’s name server is functioning properly.

452 Requested action not taken: insufficient system storage. The receiving server is overwhelmed by too many messages being sent at once, or is out of memory or storage space. Review your sending rate by looking at your mail sending queue and logs, or inform the server’s mail administrator about the storage and free memory issues.

455 Server unable to accommodate parameters. The server cannot process the command at this time. You can retry after waiting a while or contact the receiving server’s mail administrator if the error persists.

Permanent SMTP error codes

SMTP 500 reply codes are mail delivery errors of a permanent nature that are usually caused by the sender. The receiving server returns the message as a hard bounce and prefers that the sender does not try again unless the problem is fixed.

The 500 series starts with errors in issuing SMTP commands like the following examples:

500 Syntax error, command unrecognized. The receiving server does not recognize the command. If your server is using antivirus or firewall software, try disabling them before sending the message again.

501 Syntax error in parameters or arguments. The command is recognized but there are errors with its options or command arguments. This server error is sometimes returned for an invalid or incorrect email! But it may be due to your antivirus or firewall software as well.

502 Command not implemented. The command is currently not supported on the receiving server. Review your sending commands as well as your MTA’s configuration.

503 Bad sequence of commands. The receiving server recognizes the command but its parameters or options are in the wrong sequence. This sometimes appears when the email account is not authenticated. Do review your MTA’s configuration as well.

504 Command parameter not implemented. Similar to 502, the command is recognized but its parameters or options are not supported on the receiving server. You may wish to review your sending commands as well as your MTA’s configuration.

The SMTP error codes then get more serious, like being suspected of sending spam or appearing in a blocklist. As an email sender, you should pay attention to the following codes because they can affect your sending reputation and email deliverability.

541 The recipient address rejected your message. The mail from the sender has been flagged as spam or blocked by the receiving server’s rules. Find out if your IP address is in a blocklist and scan your emails for content that could trigger spam filters. If you’re clear, ask the administrator of the server nicely to put you on their safelist.

550 Requested action not taken: mailbox unavailable. The recipient’s email address does not exist. Verify the email address by making sure it is free of typos or use an email verification tool like MailerSend’s inbuilt feature.

551 User not local; please try . The recipient was not found on the server but the message will be forwarded to another email. Used for spam prevention, this error is shown when you are not authorized to relay mail through the server. Review your authentication records to see if you can use an SMTP relay.

552 Requested mail action aborted: exceeded storage allocation. The user’s mailbox is full! Besides waiting for messages to be deleted, you can try to contact them through other means and inform them that their inbox is full.

553 Requested action not taken: mailbox name not allowed. The recipient cannot be found due to errors in the email address. Check that the email addresses are correct in the To, Cc and Bcc fields.

554 Transaction failed. This is bad news! The receiving mail server will not accept your message because, for example, you’re on a blocklist. Confirm whether your IP address appears in a blocklist. If you’re good, send a request to the receiving server to put your IP in their safelist.

SMTP response codes deciphered

SMTP plays a critical role in the email infrastructure of the Internet. It’s easy to implement it in your technology stack, but when things go wrong it’s hard to decipher what those SMTP codes mean. Here’s a tip: Bookmark this page and return to it whenever you encounter one!

What are the most common SMTP error codes that you’ve come across? Tell us about it below!

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21 SMTP Response Codes That You Need To Know

An SMTP response code list can be used to help quickly determine why an email bounced, or why you’re receiving an SMTP error when sending an email.

But, if you’re not well versed in SMTP monitoring , then making sense of all of these SMTP email status codes can be overwhelming.

That’s why in this blog post, we’re going to give you 21 of the most common SMTP response codes, and a link to our free SMTP server test tool .

First, let’s start by discussing what an SMTP response code is.

The Anatomy of an SMTP Response Code

In our recent blog post, ‘ what does an SMTP error mean ’, we talk about email status codes and explain that when your email program makes a request, the receiving mail server will return a code indicating the status of the message. This code is known as an “SMTP response code.”

An SMTP response code consists of three digits:

• 1) The error code class: This first digit indicates whether or not the server accepted the command. There are five possible values ranging from 1-5. Each value defines whether the server has accepted the command, fulfilled an action, run into a temporary issue, encountered an error, etc.
• 2) Subject: The second digit in the SMTP response code gives you more information, stating whether there was a syntactic problem, connection trouble, or something else.
• 3) Detail: The last number provides even more information about the mail transfer status.

Now that you have a basic understanding of what an SMTP response code is, here are 21 common SMTP response codes, as laid out in RFC 821 (RFC stands for Request for Comments documents and has been used for more than 40 years as a way to define new standards and share technical information, such as how SMTP relaying works ).

21 Common SMTP Response Codes for Email Monitoring

Part of the inherent problem with SMTP codes is that different servers use the codes in different ways, making it impossible to state with certainty the meaning of each code. As you read through each response code below, keep in mind that the meaning could vary depending upon how your mail server was set up.

220 — SMTP Service ready.

This is a general reply stating that the SMTP server is ready to continue forward with the next command.

221 — Service closing.

This response states that the session or connection to the mail server is ending and all processes are complete. The end result could be either a success or failure.

250 — Requested action taken and completed.

This is the best message for a sender to receive because it indicates that the SMTP communication was successful. SMTP response code 250 is also the most common response code in SMTP since it is issued in response to every accepted command (likely 4 to 6 times per message).

354 — Start message input and end with …

This indicates that the server is ready to accept the message itself.

421 — The service is not available and the connection will be closed.

If you receive ‘SMTP error (421) connection server failed’, then think of this as an open ended error, which is most typically related to the destination server not being “reachable.” However, it may also mean that your sending mail server is not “reachable.” If you’re using a remote mail server for sending, you’ll want to ensure that it’s working properly and the connections are successful. You can test your SMTP connection with this tool.

450 — The requested command failed because the user’s mailbox was unavailable (for example because it was locked) try again later.

There can also be some additional meanings to this response code such as:

a) The email account no longer exists on the recipient mail server
b) The email account does not have permission to receive the email
c) The recipient mail server rejected the email due to a blacklisting or filtering

How to Fix SMTP Error 450:

1. Verify that the email account that you’re trying to reach exists
2. Check your mail server’s IP address for blacklisting. At SocketLabs, we highly recommend MX Toolbox for running a blacklist check
3. If your email appears to be spammy, then remove any spammy content from the message
4. If you’re using an attachment, then try sending the email without the attachment

SMPT Error Codes: What They Mean

Are you getting SMTP errors when sending emails? As a high volume email sender, you put a lot of effort in to making sure the emails you send are getting to the inbox successfully. If your email isn’t making it to the recipient as intended, you could be wasting a lot of time, energy, and potential business. If you are getting SMTP error codes indicating there is an underlying issue in your email delivery, you can use this guide to better understand what SMTP error codes mean and how they impact your deliverability.

What is an SMTP Error Code?

When a message is sent from a mail client to an SMTP server, the server will reply with a specific response code. This response is a normal part of the email sending process and is an essential step in getting your email to the inbox…or not.  

An SMTP response code is a three digit code that the servers use to communicate throughout the email sending process. This response code can be good, like 250 (The requested action was taken) or it can indicate an that there was a communication error. If there is a communication error, you will receive an SMTP error code —  a message that indicates the specific problem; like the end of server communication which is error code 550 (The request failed).

How to Read an SMTP Server Message

If you’re getting SMTP errors when sending emails, it’s important to understand the type of message you’re receiving before you take any action. The first number in the SMTPany three-digit code, also known as the code class, will indicate the type of response encountered by a command issued during the SMTP communication. For example, if the code starts with a 2, the server has completed the requested action.

Positive SMTP Response Codes

Generally SMTP response codes starting with 2 and 3 are normal response codes that do not indicate an error.

SMTP 200 Response Codes

• 220 – The SMTP server is ready
• 221 – The SMTP server is closing its transmission channel
• 250 – The requested action taken and completed (Good news!)

SMTP 300 Response Codes

A code starting with 3 indicates the request is understood, but the server is going to need more information to complete the task.

• 334 – Provide SMTP authentication credentials.
• 354 – Waiting for the contents of the message

SMTP Server Error Messages

SMTP 400 Error Codes

4 means the server is facing a temporary failure but the command could still be completed if tried again. Codes beginning with 4 are typically errors related to the receiving mail system. Note: Response codes beginning with 4 and 5 indicate an error has occurred.

• 421 – The service is unavailable, try again later
• 450 – The requested action was not taken because the user’s mailbox was unavailable
• 451 – The message was not sent due to a server error
• 452 – The command was aborted because there isn’t enough server storage
• 455 – The server cannot deal with the command at this time

SMTP 500 Error Codes

Then there are codes beginning with 5, perhaps the most prominent SMTP error code indicator. If the reply message starts with a 5, there is a permanent failure. Here are some examples:

• 500 – The server could not recognize the command due to a syntax error
• 501 – A syntax error was encountered in command parameters or arguments
• 502 – This command is not implemented
• 503 – The server has encountered a bad sequence of commands
• 541 – The message was rejected by the recipient address
• 550 – The requested command failed because the user’s mailbox was unavailable or the recipient server rejected the message due to high probability of spam
• 551 – The intended recipient mailbox was not available on the recipient server
• 552 – The message was not sent because the recipient mailbox does not have adequate storage
• 553 – The command was aborted because the mailbox name does not exist
• 554 – The transaction failed. No further details are given

Typically, SMTP error codes will also be followed with three more digits. This is called the SMTP enhanced status code and it provides more detail on what caused the failure. For example, in practice, an enhanced SMTP error code might look like this:

501 5.7.0 – Authentication failed.  Username or password is invalid.

It seems pretty easy, right? Just look up the SMTP error code and diagnose the problem yourself. Unfortunately, figuring out what an error code actually means can be particularly difficult. 

There is no uniform error response language that is used across all email platforms. 

This means that two different servers could both send the same error message that have completely different descriptions.  

How Do You Fix an SMTP Error?

While these are just a few SMTP error codes, there are many more that you will likely come across. Doing the research on your own and trying to figure out what an error code means and how to diagnose it can be strenuous. That’s why most high volume email senders will work with email service providers. Email service providers like SocketLabs will do the heavy lifting for you and make sure you have all of the tools you need to send your emails successfully.

If you want to learn more about SMTP error messages, visit our Message Delivery Failure Knowledge Base. If you want to learn more about improving your email deliverability with SocketLabs, visit our SocketLabs SMTP Server page.

When your transactional emails fail to get delivered, you might have an SMTP error that needs to be addressed. But some of the codes are super confusing! Read on to learn about common SMTP response codes and how you can troubleshoot delivery issues.

Simple Mail Transfer Protocol (SMTP) is the standard communication protocol for sending emails over the Internet. 

Getting started with SMTP for your transactional emails is quick and easy. You simply provide your server name, SMTP port, username and password. The SMTP server then sends emails on your behalf and takes care of email deliverability.

However, when an SMTP error prevents your emails from getting delivered, it can be difficult to understand its cryptic error codes in your delivery reports. Read on to learn how to read common SMTP response codes and how you can troubleshoot delivery errors.

What are SMTP codes

SMTP response codes are updates sent by an SMTP server during the mail delivery process. Using a 3-digit format, they are sent in response to commands issued by an SMTP client. The following diagram shows the relationship between the SMTP client and server in the mail delivery process. 

How to read SMTP codes

The SMTP specification defined basic status codes that were published in 1982. Because the codes were originally designed to report on the outcome of an SMTP command, their lack of detail made them unsuitable for delivery reports.

To compound the problem, some servers would assign the same error code to different delivery failures. Others would assign a code that only explained its meaning in the text description. Inconsistent application of the codes meant it was difficult to resolve mail sending issues!

Enter the enhanced status codes. They extend the basic status codes by providing more detail about the cause of the email delivery failure. Similarly, they also consist of 3-digit numbers separated by decimals. Let’s take a look at how to read them next.

Basic status codes

The first digit of a basic status code tells the sending server whether the response is good, bad or incomplete:

• 2: Positive Completion Reply. The requested action has been successfully completed.

• 3: Positive Intermediate Reply. The command is accepted but more information is needed before proceeding.

• 4: Transient Negative Completion Reply. The command was not accepted and no action was taken. This error is seen as temporary (soft bounce) and the sender can try again later.

• 5: Permanent Negative Completion Reply. The command was not accepted and no action was taken. This is a permanent error (hard bounce) and the sender should not repeat the command.

The second digit puts the SMTP response into a specific category:

• 0: Syntax. Responses about command or parameters.

• 1: Information. Responses to requests for more information.

• 2: Connections. Responses about the transmission channel.

• 5: Mail system. Responses about the status of the receiving mail server.

The third digit goes into more detail within the response category.

For example, if you telnet to MailerSend’s SMTP server and introduce yourself with the HELO command, you will see the following exchange:

HELO mailersend.net

250 smtp.mailersend.net Nice to meet you, [8.38.148.103]

The 250 response code means that it is a positive completion reply about the status of the mail system, i.e. the SMTP server has acknowledged your IP address and is waiting for your next command.

Enhanced status codes

Like the basic status codes, the first digit defines the class to which the code belongs:

• 2: Success! The requested mail action is okay and completed.

• 4: Persistent Transient Failure. Temporary conditions will cause the message to be delayed or abandoned.

• 5: Permanent Failure. The message in its current form cannot be delivered.

The second digit identifies the subject of the SMTP reply code:

• 0: Other or Undefined Status

• 1: Addressing Status

• 2: Mailbox Status

• 3: Mail System Status

• 4: Network and Routing Status

• 5: Mail Delivery Protocol Status

• 6: Message Content or Media Status

• 7: Security or Policy Status

Finally, the third digit provides more detail and it is dependent on the class and the subject.

For example, if you specify the recipient of the email with the RCPT TO command, the SMTP server may respond with 250 2.1.5 Recipient OK. This code means that not only is your command successful, the address of the recipient at the server has been accepted as well.

Common SMTP codes and how to troubleshoot them

The current SMTP standard defines the response codes but they are applied differently between SMTP servers. Not every mail server is configured in the same way and thus there will be differences in how mail delivery failures are interpreted.

General SMTP status codes

SMTP codes that start with 2 and 3 are general status messages that are sent in reply to requests from the sending server. They do not indicate errors in the mail delivery but are used as prompts in the exchange between the mail servers.

• 214 Help message. A response to the HELP command that usually includes a link or URL to the FAQ page.

• 220 SMTP Service ready. The receiving server is ready for the next command.

• 221 Service closing transmission channel. The receiving server is closing the SMTP connection.

• 235 2.7.0 Authentication succeeded: The sending server’s authentication is successful.

• 250 Requested mail action okay, completed. Success! The email was delivered.

• 251 User not local; will forward to <forward-path>. The receiving server doesn’t recognize the recipient but it will forward it to another email address.

• 252 Cannot VRFY user, but will accept message and attempt delivery. The receiving server doesn’t recognize the recipient but it will try to deliver the email anyway.

• 354 Start mail input. The email “header” has been received, the server is now waiting for the “body” of the message.

Temporary SMTP error codes

SMTP 400 codes are transient errors and are usually related to issues at the receiving mail server. Messages are returned as a soft bounce and the sender may try again after, for example, reducing the size of the attachments in the email.

• 421 Service not available, closing transmission channel. The receiving server or sending server is not reachable but another mail delivery will be attempted. If you are using a remote server like MailerSend’s SMTP relay to send emails, test that you can connect to it successfully. Otherwise, you may wish to check the receiving server’s availability.

• 450 Requested mail action not taken: mailbox unavailable. This error could mean that the recipient does not exist, the mailbox does not have permission to receive the email, or the message was rejected due to a blocklist or filter.

  To fix this, you should confirm the recipient’s email address, see if your IP address is on a blocklist, test your email for spam-like content, or try sending an email without an attachment.

• 451 Requested action aborted: error in processing. The receiving server is unable to process the message due to email authentication rules. If Sender Policy Framework (SPF) is being used, for example, you should confirm that a DNS lookup can be made for your sending domain and that the domain’s name server is functioning properly.

• 452 Requested action not taken: insufficient system storage. The receiving server is overwhelmed by too many messages being sent at once, or is out of memory or storage space. Review your sending rate by looking at your mail sending queue and logs, or inform the server’s mail administrator about the storage and free memory issues.

• 455 Server unable to accommodate parameters. The server cannot process the command at this time. You can retry after waiting a while or contact the receiving server’s mail administrator if the error persists.

Permanent SMTP error codes

SMTP 500 reply codes are mail delivery errors of a permanent nature that are usually caused by the sender. The receiving server returns the message as a hard bounce and prefers that the sender does not try again unless the problem is fixed.

The 500 series starts with errors in issuing SMTP commands like the following examples:

• 500 Syntax error, command unrecognized. The receiving server does not recognize the command. If your server is using antivirus or firewall software, try disabling them before sending the message again.

• 501 Syntax error in parameters or arguments. The command is recognized but there are errors with its options or command arguments. This server error is sometimes returned for an invalid or incorrect email! But it may be due to your antivirus or firewall software as well.

• 502 Command not implemented. The command is currently not supported on the receiving server. Review your sending commands as well as your MTA’s configuration.

• 503 Bad sequence of commands. The receiving server recognizes the command but its parameters or options are in the wrong sequence. This sometimes appears when the email account is not authenticated. Do review your MTA’s configuration as well.

• 504 Command parameter not implemented. Similar to 502, the command is recognized but its parameters or options are not supported on the receiving server. You may wish to review your sending commands as well as your MTA’s configuration.

The SMTP error codes then get more serious, like being suspected of sending spam or appearing in a blocklist. As an email sender, you should pay attention to the following codes because they can affect your sending reputation and email deliverability.

• 541 The recipient address rejected your message. The mail from the sender has been flagged as spam or blocked by the receiving server’s rules. Find out if your IP address is in a blocklist and scan your emails for content that could trigger spam filters. If you’re clear, ask the administrator of the server nicely to put you on their safelist.

• 550 Requested action not taken: mailbox unavailable. The recipient’s email address does not exist. Verify the email address by making sure it is free of typos or use an email verification tool like MailerSend’s inbuilt feature.

• 551 User not local; please try <forward-path>. The recipient was not found on the server but the message will be forwarded to another email. Used for spam prevention, this error is shown when you are not authorized to relay mail through the server. Review your authentication records to see if you can use an SMTP relay.

• 552 Requested mail action aborted: exceeded storage allocation. The user’s mailbox is full! Besides waiting for messages to be deleted, you can try to contact them through other means and inform them that their inbox is full.

• 553 Requested action not taken: mailbox name not allowed. The recipient cannot be found due to errors in the email address. Check that the email addresses are correct in the To, Cc and Bcc fields.

• 554 Transaction failed. This is bad news! The receiving mail server will not accept your message because, for example, you’re on a blocklist. Confirm whether your IP address appears in a blocklist. If you’re good, send a request to the receiving server to put your IP in their safelist.

SMTP response codes deciphered

SMTP plays a critical role in the email infrastructure of the Internet. It’s easy to implement it in your technology stack, but when things go wrong it’s hard to decipher what those SMTP codes mean. Here’s a tip: Bookmark this page and return to it whenever you encounter one!

What are the most common SMTP error codes that you’ve come across? Tell us about it below!

From Wikipedia, the free encyclopedia

This is a list of Simple Mail Transfer Protocol (SMTP) response status codes. Status codes are issued by a server in response to a client’s request made to the server.

Unless otherwise stated, all status codes described here is part of the current SMTP standard, RFC 5321. The message phrases shown are typical, but any human-readable alternative may be provided.

Basic status code[edit]

A «Basic Status Code» SMTP reply consists of a three digit number (transmitted as three numeric characters) followed by some text. The number is for use by automata (e.g., email clients) to determine what state to enter next; the text («Text Part») is for the human user.

The first digit denotes whether the response is good, bad, or incomplete:

• 2yz (Positive Completion Reply): The requested action has been successfully completed.
• 3yz (Positive Intermediate Reply): The command has been accepted, but the requested action is being held in abeyance, pending receipt of further information.
• 4yz (Transient Negative Completion Reply): The command was not accepted, and the requested action did not occur. However, the error condition is temporary, and the action may be requested again.
• 5yz (Permanent Negative Completion Reply): The command was not accepted and the requested action did not occur. The SMTP client SHOULD NOT repeat the exact request (in the same sequence).

The second digit encodes responses in specific categories:

• x0z (Syntax): These replies refer to syntax errors, syntactically correct commands that do not fit any functional category, and unimplemented or superfluous commands.
• x1z (Information): These are replies to requests for information.
• x2z (Connections): These are replies referring to the transmission channel.
• x3z : Unspecified.
• x4z : Unspecified.
• x5z (Mail system): These replies indicate the status of the receiver mail system.

Enhanced status code[edit]

The Basic Status Codes have been in SMTP from the beginning, with RFC 821 in 1982, but were extended rather extensively, and haphazardly so that by 2003 RFC 3463 rather grumpily noted that: «SMTP suffers some scars from history, most notably the unfortunate damage to the reply code extension mechanism by uncontrolled use.»

RFC 3463 defines a separate series of enhanced mail system status codes which is intended to be better structured, consisting of three numerical fields separated by «.», as follows:

class "." subject "." detail

  class   = "2" / "4" / "5"

  subject = 1 to 3 digits

  detail  = 1 to 3 digits

The classes are defined as follows:

• 2.XXX.XXX Success: Report of a positive delivery action.
• 4.XXX.XXX Persistent Transient Failure: Message as sent is valid, but persistence of some temporary conditions has caused abandonment or delay.
• 5.XXX.XXX Permanent Failure: Not likely to be resolved by resending the message in current form.

In general the class identifier MUST match the first digit of the Basic Status Code to which it applies.^[1]

The subjects are defined as follows:

• X.0.XXX Other or Undefined Status
• X.1.XXX Addressing Status
• X.2.XXX Mailbox Status
• X.3.XXX Mail System Status
• X.4.XXX Network and Routing Status
• X.5.XXX Mail Delivery Protocol Status
• X.6.XXX Message Content or Media Status
• X.7.XXX Security or Policy Status

The meaning of the «detail» field depends on the class and the subject, and are listed in RFC 3463 and RFC 5248.

A server capable of replying with an Enhanced Status Code MUST preface (prepend) the Text Part of SMTP Server responses with the Enhanced Status Code followed by one or more spaces. For example, the «221 Bye» reply (after QUIT command) MUST be sent as «221 2.0.0 Bye» instead.^[1]

The Internet Assigned Numbers Authority (IANA) maintains the official registry of these enhanced status codes.^[2]

Common status codes[edit]

This section list some of the more commonly encountered SMTP Status Codes. This list is not exhaustive, and the actual text message (outside of the 3-field Enhanced Status Code) might be different.

— 2yz Positive completion[edit]

211 System status, or system help reply

214 Help message (A response to the HELP command)

220 <domain> Service ready

221 <domain> Service closing transmission channel

221 2.0.0 Goodbye ^[1]

235 2.7.0 Authentication succeeded ^[3]

240 QUIT

250 Requested mail action okay, completed

251 User not local; will forward

252 Cannot verify the user, but it will try to deliver the message anyway

— 3yz Positive intermediate[edit]

334 (Server challenge — the text part contains the Base64-encoded challenge) ^[3]

354 Start mail input

— 4yz Transient negative completion[edit]

«Transient Negative» means the error condition is temporary, and the action may be requested again. The sender should return to the beginning of the command sequence (if any).

The accurate meaning of «transient» needs to be agreed upon between the two different sites (receiver- and sender-SMTP agents) must agree on the interpretation. Each reply in this category might have a different time value, but the SMTP client SHOULD try again.

421 Service not available, closing transmission channel (This may be a reply to any command if the service knows it must shut down)

432 4.7.12 A password transition is needed ^[3]

450 Requested mail action not taken: mailbox unavailable (e.g., mailbox busy or temporarily blocked for policy reasons)

451 Requested action aborted: local error in processing

451 4.4.1 IMAP server unavailable ^[4]

452 Requested action not taken: insufficient system storage

454 4.7.0 Temporary authentication failure ^[3]

455 Server unable to accommodate parameters

— 5yz Permanent negative completion[edit]

The SMTP client SHOULD NOT repeat the exact request (in the same sequence). Even some «permanent» error conditions can be corrected, so the human user may want to direct the SMTP client to reinitiate the command sequence by direct action at some point in the future.

500 Syntax error, command unrecognized (This may include errors such as command line too long)

500 5.5.6 Authentication Exchange line is too long ^[3]

501 Syntax error in parameters or arguments

501 5.5.2 Cannot Base64-decode Client responses ^[3]

501 5.7.0 Client initiated Authentication Exchange (only when the SASL mechanism specified that client does not begin the authentication exchange) ^[3]

502 Command not implemented

503 Bad sequence of commands

504 Command parameter is not implemented

504 5.5.4 Unrecognized authentication type ^[3]

521 Server does not accept mail ^[5]

523 Encryption Needed ^[6]

530 5.7.0 Authentication required ^[3]

534 5.7.9 Authentication mechanism is too weak ^[3]

535 5.7.8 Authentication credentials invalid ^[3]

538 5.7.11 Encryption required for requested authentication mechanism^[3]

550 Requested action not taken: mailbox unavailable (e.g., mailbox not found, no access, or command rejected for policy reasons)

551 User not local; please try <forward-path>

552 Requested mail action aborted: exceeded storage allocation

553 Requested action not taken: mailbox name not allowed

554 Transaction has failed (Or, in the case of a connection-opening response, «No SMTP service here»)

554 5.3.4 Message too big for system ^[4]

556 Domain does not accept mail ^[5]

Example[edit]

Below is an example SMTP connection, where a client «C» is sending to server «S»:

S: 220 smtp.example.com ESMTP Postfix
C: HELO relay.example.com
S: 250 smtp.example.com, I am glad to meet you
C: MAIL FROM:<bob@example.com>
S: 250 Ok
C: RCPT TO:<alice@example.com>
S: 250 Ok
C: RCPT TO:<theboss@example.com>
S: 250 Ok
C: DATA
S: 354 End data with <CR><LF>.<CR><LF>
C: From: "Bob Example" <bob@example.com>
C: To: Alice Example <alice@example.com>
C: Cc: theboss@example.com
C: Date: Tue, 15 Jan 2008 16:02:43 -0500
C: Subject: Test message
C: 
C: Hello Alice.
C: This is a test message with 5 header fields and 4 lines in the message body.
C: Your friend,
C: Bob
C: .
S: 250 Ok: queued as 12345
C: QUIT
S: 221 Bye
{The server closes the connection}

And below is an example of an SMTP connection in which the SMTP Server supports the Enhanced Status Code, taken from RFC 2034:

S: 220 dbc.mtview.ca.us SMTP service ready
C: EHLO ymir.claremont.edu
S: 250-dbc.mtview.ca.us says hello
S: 250 ENHANCEDSTATUSCODES
C: MAIL FROM:<ned@ymir.claremont.edu>
S: 250 2.1.0 Originator <ned@ymir.claremont.edu> ok
C: RCPT TO:<mrose@dbc.mtview.ca.us>
S: 250 2.1.5 Recipient <mrose@dbc.mtview.ca.us> ok
C: RCPT TO:<nosuchuser@dbc.mtview.ca.us>
S: 550 5.1.1 Mailbox "nosuchuser" does not exist
C: RCPT TO:<remoteuser@isi.edu>
S: 551-5.7.1 Forwarding to remote hosts disabled
S: 551 5.7.1 Select another host to act as your forwarder
C: DATA
S: 354 Send message, ending in CRLF.CRLF.
 ...
C: .
S: 250 2.6.0 Message accepted
C: QUIT
S: 221 2.0.0 Goodbye
{The server closes the connection}

References[edit]