Messages from internal exchange server to office 365 not flowing

Question

I have hundreds of emails sent from internal systems (not user mailboxes) addressed to internal email addresses hosted in exchange online that are stuck. The oldest is 2 days old but all new messages sent from automated email systems and from our printers are just adding to the queues.
Mail from EOL to EOL is flowing fine and mail from external addresses to EOL is fine and outbound mail from EOL is fine.

The stuck messages have a variety of errors listed, the oldest messages all say ;0;[{LED=400 4.4.7 Message delayed};{MSG=};{FQDN=};{IP=};{LRT=}];0;;0
others say Messaged deferred by categorizing agent.

All Exchange services are running, the server was rebooted this morning but the messages are still stuck.

The oldest message is addressed to our IT helpdesk which is receiving messages without issue from internal and external senders.

The certificate for the Exchange server was renewed a week before this started, the new one has SMTP and IIS listed as the covered services whereas the expired one had just SMTP listed, the old certificate is still installed and shows date invalid as the status, but everything was working fine for a week after the new one was installed.

What could be the cause of this issue?

Answer 1

Hi, known issue:

see the the blog post on how to fix it -

https://techcommunity.microsoft.com/t5/exchange-team-blog/email-stuck-in-transport-queues/ba-p/3049447

Fix included in blog post

Answer 2

that did the job. It took some time to complete but once it had downloaded, updated and restarted the services the mail queues cleared pretty quickly.
Thanks