Permanent failure — do not retry
SMTP 554 5.6.0
message malformed
SMTP 554 5.6.0 means the receiving server rejected the message because its content or structure is malformed. RFC 3463 X.6.0 covers "other or undefined media error", and Gmail returns "Email message is malformed. Not accepted." The fault is in how the message was built, not in the address.
How it appears
554 5.6.0 Email message is malformed. Not accepted.
554 5.6.0 Message contains invalid header
What causes it
- Broken MIME structure, such as a missing boundary or bad encoding.
- Duplicate or invalid headers, for example two From headers.
- Non-ASCII characters in headers without proper encoding.
- A buggy template engine or mail library.
How to fix it
In order — try the first one first.
- Send a test to yourself and inspect the raw source for broken MIME parts or duplicate headers.
- Update or switch the library that builds your messages.
- Encode non-ASCII header values properly.
- Do not remove the recipient. Fix the message and resend.
Common questions
- What does SMTP 554 5.6.0 mean?
- SMTP 554 5.6.0 means the receiving server rejected the message because its content or structure is malformed. RFC 3463 X.6.0 covers "other or undefined media error", and Gmail returns "Email message is malformed. Not accepted." The fault is in how the message was built, not in the address.
- Should I retry a 554 5.6.0 error?
- No. 554 is a permanent failure, so the message will never be delivered no matter how many times you try. Remove the address — repeated sends to permanent failures are one of the main signals mailbox providers use to throttle a sender.
- How do I fix SMTP 554 5.6.0?
- Send a test to yourself and inspect the raw source for broken MIME parts or duplicate headers.
Other 554 replies
Last reviewed 2026-09-11