4.4.7 Message expired
Sample bounce line
451 4.4.7 Message expired; the message in the queue expired and could not be delivered before the message expiration time
What it means
The message sat in the queue until it expired. Microsoft's explanation is that the sending server kept trying to relay or deliver it and the attempt never completed in time, which can also indicate a header limit on the remote server or a protocol timeout. It is a temporary code, so earlier attempts were deferred rather than rejected.
Why it happens
- The receiving server was unreachable or refusing connections for the whole retry window.
- A recipient domain with broken MX records, where every attempt failed to resolve.
- Too many recipients in the message header for the receiving host to accept.
- A protocol timeout partway through the conversation, repeated on every retry.
How to fix it
- Check the recipient address and the recipient domain's MX records before resending.
- Test whether the receiving server accepts a connection at all right now; if it does, resending will queue the message again and it will go.
- Reduce the number of recipients per message for the host that is failing.
- If this affects one destination only, the problem is at that destination and the sender can do little but wait and retry.
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