Hello All,

Hoping someone can shed some light on issues I am experiencing. Currently mail is being delayed, and I cannot figure out why. Mail being delayed is not specific senders, it just happens randomly, but frequently enough that causing us many issues. We have 2 Exchange 2013 servers along with office 365 (1 user) and EOP. We have no anti-malware, antivirus, or anti-spam on the server, just what is provided to us by Exchange Online Protection. A lot of the errors we are seeing while doing a message trace through outlook.com shows an error code 451 4.4.0 SMTPSEND.SuspiciousRemoteServerError. remote server disconnected abruptly. Retry will be delayed. We have double-checked what feels like everything in the system, and still cannot figure out the root cause of the issue. We have contacted Microsoft and they blame us or the senders. Any help would be MUCH appreciated! 

I would start with very basic level to see with message tracking and message header for the delayed samples and see where exactly it is getting delayed. if it is between your on-premises then it is with you if EOP/O365 mailbox involved then you need to find why exactly...

BTW, when exactly you get 451 4.4.0 SMTPSEND.SuspiciousRemoteServerError message, while receiving email from outside EOP or O365 user or while sending it to them?

 

I see the 451 4.4.0 in both the SMTP logs on the server and the message trace feature on outlook.com. I am seeing this delay when we are receiving messages from outside EOP/O365 coming in.