Email stuck in Exchange Message queue: SMTPSEND.SuspiciousRemoteServerError; remote server disconnected abruptly – returned 451 4.4.0

After migrating on new Exchange environment I have noticed that couple of emails are stuck in Message queue with following error:

 

SMTPSEND.SuspiciousRemoteServerError; remote server disconnected abruptly – returned 451 4.4.0

 

After some troubleshooting, we found out that our Firewall had IPS feature enabled on SMTP traffic. Once disabled, emails successfully went through.

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