Enhanced SMTP status code · RFC 3463 · IANA
5.4.300 Message expired
The message was not delivered in time: the recipient's server did not answer or an error occurred.
- Class: Permanent failure
Class 5: a permanent failure. Resending the same message will not help: something about the message, the sender or the destination must change (RFC 3463 §3.1).
- Subject: X.4 · Network and routing
Network and routing status (DNS included); under the control of the destination's or an intermediate server's administrator.
How 5.4.300 appears at providers
The exact text from the provider's documentation, as you see it in the bounce, and what it means.
Microsoft 365 / Exchange Online
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5.4.300
Message expired
The message was not delivered in time: the recipient's server did not answer or an error occurred. Check with SMTP server check.
What you can check here
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026