I know this error message is discussed on other topics on several different forums including TechNet but I ended up opening this as the suggestions that solved the problems with the previous threads dos not work on mine. 

We have a ticketing software that runs on client computers which sends mails to local Exchange server for notifications using port 25. 

Approximately 1 out of 5 mails end up getting the "451 4.7.0 Temporary server error. Please try again later. PRX5" error. I have confirmed this by running Wireshark on the clients, saw the exact error message. This has been replicated with several client machines running Win7 & 8.

I have tried several solutions to this, one of them being this:

http://www.techieshelp.com/exchange-2013-451-4-7-0-temporary-server-error-please-try-again-later-prx5/

I still get the same error from time to time after doing this, so it is not the issue. All the records in local hosts file, receive connector is listening to port 25 on all IPv4 addresses. 

The server has a single NIC and the DNS lookups are done through that single NIC, not through "All available adapters" as set by default on ECP.  

Almost everyone seems to be convinced that this is related to DNS but looking at how fast I get the error message, it doesn't look like it's waiting for a timeout (maybe unrelated, just an observation). Do you think it is possible that the DNS servers themselves are causing this issue? 

Also, I see Event 5504 on the primary dns server almost all of them coming from the mail server, saying that the DNS query was invalid. It may be related. 

Additional Notes:

- CU6 is applied but did not fix the issue. 

- None of these have solved it for me: http://www.servolutions.com/support/articles/Temporaryservererror2013.htm 

- Receive Connectors do listen to separate ports 25 & 2525 -> There is a single instance of both Transport Service and Edge Transport service respectively. Anonymous users are accepted in both Receive connectors. 

• Edited by mbaybarsk Tuesday, November 18, 2014 8:28 PM

Thank you for your post.

This is a quick note to let you know that we are performing research on this issue.

Thank you for your post.

This is a quick note to let you know that we are performing research on this issue.

Hi,

change from all available network adapter to one adapter in the DNS Lookups config

Best regards

Hi,

change from all available network adapter to one adapter in the DNS Lookups config

Best regards

• Edited by mbaybarsk Wednesday, November 19, 2014 1:24 AM

Mbaybarks,

did you managed to fix this issue? Went through the same attempts you did, but still get this error from time to time

We have the same issue here.

Been through all the same recommendations - host file, setting internal/external dns on transport and frontendtransportservices, change all adapters to specific one and ip of exchange server.

Our single Exchange 2013 CU8 server is running on a 2012R2 Hyper-V VM. The domain is 2003 and 2 DNS servers are on 2003.

The PRX5 is intermittent and seems to affect occasional incoming smtp mail but also we have a relay connector setup for a CRM application to email thorugh as it does not support Exchange 2013 authentication. The big gripe for us is that the CRM software thinks the emails are going out when some are not which is only discovered either by telephoning the recipient or constantly checking Program Files\Microsoft\Exchange Server\V15\TransportRoles\Logs\FrontEnd\ProtocolLog\SmtpReceive for PRX5 errors.

I hope someone finds a solution for this.