5.4.300 Microsoft bounce: not delivered in time
What 5.4.300 means
The message was not delivered in time: the destination server did not respond, or an error occurred, and the bounce could not be sent back to you.
Message text
Message expired
Common causes of 5.4.300
- Destination mail server did not respond within the delivery window
- An error on the receiving side prevented delivery and the NDR could not be returned
How to fix 5.4.300
- Check that the recipient domain answers on its MX hosts with MX lookup.
- Send the message again once the recipient server is reachable.
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How to read 5.4.300
The second part, 5.4.300, is an enhanced status code (RFC 3463 §2) written class.subject.detail. Class 5 is a Permanent failure (5xx): the command was rejected and the sender should not repeat it unchanged (RFC 5321 §4.2.1; RFC 3463 §2).. Subject 4 is network and routing: a problem reaching or routing to the destination. The detail number refines the subject, and providers may define details of their own, which is why one code can read differently at Gmail, Microsoft and Yahoo.
Do not keep resending the same message: a permanent failure repeats until the cause is fixed. Correct the cause, then send a new message.
Official source: learn.microsoft.com. Last verified 2026-10-20.
Related guides
- How to set up MX records (priorities, null MX, migration)
- MX host does not resolve: causes and how to fix it
- Null MX (RFC 7505): publish it for domains with no mail
Related codes
- 5.4.4 SMTP: unable to route
- 5.4.1 Microsoft: relay access denied
- 5.4.1 Microsoft: recipient rejected
- 5.4.14 Microsoft: routing loop
- 5.4.8 Microsoft: mx not in mta-sts policy
- 550 5.4.5 Gmail: daily sending limit
FAQ
What does 5.4.300 mean?
The message was not delivered in time: the destination server did not respond, or an error occurred, and the bounce could not be sent back to you. This is a permanent failure.
How do I fix 5.4.300?
Check that the recipient domain answers on its MX hosts with MX lookup. Send the message again once the recipient server is reachable.
Is 5.4.300 permanent, and should I retry?
5.4.300 is a permanent failure, so retrying the same message will fail again. Fix the cause first, then send a new message.