Enhanced SMTP status code · RFC 3463 · IANA
5.6.0 Content or media problem
Something in the message content made it undeliverable, with no more precise cause.
- 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 5.6.0 appears at providers
The exact text from the provider's documentation, as you see it in the bounce, and what it means.
Gmail
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554 5.6.0
Email message is malformed. Not accepted.
The message does not follow the RFC 5322 format. Check with Header analyzer.
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