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.