Temporary server error please try again later prx5

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451 4.7.0 Temporary server error. Please try again later. PRX5
And in the connectivity logs you may see
NS server returned ErrorRetry reported by 0.0.0.0. [Domain:Result] = server.domain.com:ErrorRetry
(The DNS query for  'Undefined':'internalproxy':'00000000-0000-0000-0000-000000000000' failed with error : ErrorRetry)

192.168.16.1 server
192.168.16.1 server.domain.local

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Folks-

I’ve got one that is starting to drive me nuts. I recently replaced our Exchange 2013 server with an Exchange 2016 server (about 6 months ago) and ever since I have had random intermittent problems with inbound email. Several of our users get email notifications from one of our tools and they correspond to new items in a production queue (the sender is outside of our network). Every once in a while the emails get delayed by a few minutes. This only happened once or twice every 20 — 50 notifications at first, and the delays were just 2-5 minutes so I figured something on the sender end or on the internet was the cause of the delay. Recently it has gotten to almost 10% of the emails are getting delayed and it is occasionally delaying upwards of 15 minutes and longer. No one else is reporting any problems and inside organization email is not getting delayed at all.

I look into the server and all of the logs I see indicate that it is processing messages as soon as it receives them. Check system load and find nothing out of the ordinary. Less than 50% load on either CPU or memory. Nothing in the event logs that relate to my issue.

Next is to look at our Ironport spam filter. Aha! Error messages.

SMTP delivery connection (DCID 1978258) opened from Cisco IronPort interface X.X.X.X to IP address Y.Y.Y.Y on port 25.
(DCID 1978258) Delivery started for message 5469942 to name@server.com.
(DCID 1978258) Message 5469942 to name@server.com delayed. Reason: 4.3.0 - Other mail system problem ('451', ['4.7.0 Temporary server error. Please try again later. PRX5 ']) []
Message 5469942 to name@server.com pending until Tue Jun 20 14:39:28 2017 as per bounce profile Default. 

This pattern can be found on all of the delayed emails. Sometimes the delays are as short as 2-3 minutes, and sometimes as long a 1 hour. Sometimes they repeat multiple times for a single email before it finally gets delivered. All messages get delivered eventually if they go through the Ironport.

I notice that this is most commonly happening to the notification emails but other users are getting this, but less frequently. It happens more often in the middle of the workday, but that also matches our email flow. The errors also happen even when the system is not busy.

Dig around online for more information and find this: https://social.technet.microsoft.com/Forums/exchange/en-US/be568eab-45c1-4648-88df-99f4939dd845/mailflow-broken-error-451-470

Looks promising, appears to describe something really close to what I am encountering, claims to be for Exchange 2013, but 2016 is very similar and relatively new. Follow the steps. Just as I complete the powershell commands an outbound email appeared to get stuck and was repeatedly getting resent so I decided to reboot the server rather than simply restart the relevant services. Watch the server closely and am rewarded with 24 hours of relative peace and all of the users happy to be getting all of their emails. Thinks it’s all over and get on with my life.

The following day its back and happening to about 10% of the emails again. The delays are not as bad as before. Check to see if I missed any steps in the guide I followed. Remove the secondary DNS from the Exchange server completely. Restarted the services. Oddly the outbound email getting stuck in a loop happened again with the exact same user sending an email to the exact same person (actually a salesforce account for a vendor) so I figure a reboot is needed. I doubt that is actually relevant as that is our only outbound problem and no one else has had this same problem before. All of the problems go away again, this time for the weekend (we are closed for the weekend so nothing to monitor).

Of course the problem comes back at a moment that I’m dealing with another urgent issue so I quickly reboot the server. No configuration changes made. Problem goes away again.

At this point I am very frustrated. I can get a few days of peace by rebooting the server, but I can’t get it solved. I’m at a small business, so no support contracts in my back pocket.

Server — Virtualized vSphere with 6 processors & 32 GB RAM.

Exchange 2016 CU4 running on Server 2012 R2.

There are only 80 users so the load isn’t very heavy most of the time.

Single server with all roles, no DAG.

Almost forgot, inbound mail logs only show when the messages have been received. So I don’t know where to find out why it is being delayed.

Way old edit: Found out that an antivirus product on the server was not compatible with Exchange 2016 and was the ultimate source of the failures. Changed that out and the problem is solved.