We are receiving the following error when sending emails from external addresses to distribution groups. 

SMTP Error 451 4.7.0 Temporary server error. Please try again later. PRX1

The option to allow unauthenticated users to send email us correctly set. 

2 CAS Servers

2 Mailbox servers

Exchange 2013 CU 8

was not able to find to much documentation for this error. Most of what I found applied to a similar error with the PRX5 code at the end. 

please enable the check mark require that all senders are authenticated option 
in message delivery restrictions of that distribution group
This should solve your problem

Hi,

According to your description, I understand that external user failed send message to DG with error SMTP Error 451 4.7.0 Temporary server error. Please try again later. PRX1.
If I misunderstand your concern, please do not hesitate to let me know.

I want to double confirm whether all external account cannot send message to your environment, or this external user can send message to a single user.
If all inbound mail flow failed, it indicate that inbound mail flow failed. Please refer to Mail flow.
If this issue only occur on this DG, please try suggestion as Sathish provided.
Please post the result, its more helpful for further troubleshooting.

For error SMTP Error 451 4.7.0 Temporary server error. Please try again later. PRX1, it indicate that the encoded SMTP response for when proxy fails because destinations were not provided.
Meanwhile, please use Protocol log and Message tacking log to get more details about this issue.

Thank you for reply however this option is already allowed. 

Allen Thank you for your response. 

Issue is external senders to DGs. However DGs are set to allow unauthenticated 

Can you provide paths for protocol and message tracking log?

Hi 

"SMTP Error 451 4.7.0 Temporary server error. Please try again later" I suspect some DNS issue on the exchange based on the above error. You can possibly check the dns settings 

Run the below command to check the values of dns servers 
Get-Transportserver [YOUREXCHANGESERVER] : Format-List internaldnsservers, externaldnsservers

If you find the values to be empty then run the below command 
Set-TransportService exchange2013 -ExternalDnsServer 10.10.100.10, 10.10.100.11
Set-TransportService exchange2013 -InternalDnsServer 10.10.100.10, 10.10.100.11

Set-FrontEndTransportService exchange2013 -ExternalDnsServer 10.10.100.10, 10.10.100.11
Set-FrontEndTransportService exchange2013 -InternalDnsServer 10.10.100.10, 10.10.100.11

Make sure transport service and frontendtransport service both has the same DNS server values

If you are looking for the protocol logs and message tracking logs path you can refer my article

http://exchangequery.com/2014/02/21/analyzing-the-protocol-logs-and-message-tracking-logs-in-exchange-2013/

Hi jcot,

Any updates about this question?