Enhanced SMTP status code · RFC 3463 · IANA
5.6.240 One of MIME Headers is not allowed
The message contains a MIME header that is not allowed.
- 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.6 · Content
Problems with the message content (types, conversions); up to both sender and recipient.
How 5.6.240 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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550 5.6.240
One of MIME Headers is not allowed.
The message contains a MIME header that is not allowed.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026