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Mail Queuing and not being delivered on Ex2016 to EXO
Hello. I have an issue with mail building up in our Ex2016 queues. This transports from our 3rd party gateway to Exchange online as part of a hybrid setup. Our mailboxes are all in EXO. Restarting th...
FuriousHaggisIt's always the network
ANSWER: Firewall was blocking connection from Onprem to EXO which was resolving to an IP block we've not seen before. Adding this to the FW rule resolved. I found 451 4.4.397 errors in the SMTP logs to get me there. The IP block was in MS's documentation so was either missed when we set up or added afterwards.
FuriousHaggisIt's always the network