Custom mail server - emails stuck in outgoing (451 4.7.500 Server busy)

Question

We have a self-hosted email server running in our business. There have been service interruptions, and outgoing mails to outlook are stuck in mail queue (mailq), with error message:

451 4.7.500 Server busy

This problem has been going on intermittently since October 12.

Any updates on this issue?

Answer 1

Hi @Karl

Based on the error message and when it occurred, it appears to be a known issue.

The error message you're seeing (451 4.7.500 Server busy. Please try again later) is a temporary one, but it can be quite persistent if Microsoft's servers have identified any suspicious patterns or issues with emails coming from your server.

You can try the methods mentioned in this thread to fix this issue:

Deferred mail with error 451 4.7.500 Server busy when sending mail to domains hosted Microsoft office 365 - Microsoft Q&A.

Hope this helps!

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