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.
Should you remove the address?
Keep the address; fix the sending side
Keep it. The message is the problem.
Block. Retrying changes nothing until the cause on your side is fixed.
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.
Who sends it
Gmail documents it; most servers reject malformed messages similarly.
Gmail
Reply text Email message is malformed. Not accepted.
Gmail refers to its sender guidelines and RFC 5322.
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.
- Is 554 5.6.0 a hard bounce or a soft bounce?
- 554 5.6.0 is a block rather than a classic hard or soft bounce. The rejection is about your sending (authentication, reputation, content or the recipient's policy), not about whether the address exists. Retrying changes nothing until the cause on your side is fixed.
- Should I remove the address after a 554 5.6.0 bounce?
- Keep it. The message is the problem.
- Will verifying my list prevent 554 5.6.0?
- No. This is about message structure.
Clean the list before the next send.
A bounce report only tells you about the addresses you already mailed. The rest of the list has the same kinds of dead mailboxes in it, and each one becomes a hard bounce recorded against your domain the moment you send.
- A real SMTP mailbox check on every row, before anything is sent
- Dead and mistyped addresses split out, so they never reach your sending tool
- Catch-all, disposable and role addresses flagged alongside
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Other 554 replies
Sources
- Source: Google Workspace Admin Help: Gmail SMTP errors and codes
- Source: RFC 3463: Enhanced Mail System Status Codes
Provider wording is quoted only where the provider publishes it.
Last reviewed 2026-10-22