SMTP enhanced status code 5.6.0

What the registry says

In practice

X.6.0 is the generic content code. Gmail returns 554 5.6.0 for a malformed message that is not accepted. Malformed MIME boundaries, broken encodings and invalid headers produced by template tools are common causes. Send the same message to a test inbox and inspect the raw source. Messages built by hand-written code are the usual source, especially when headers are joined without proper line folding or the multipart boundary appears inside the content.

What the sender should do

Check the message's MIME structure and headers for errors. Sending a plain, well-formed test message is a quick way to confirm.

How providers use X.6.0

Quoted from each provider’s own error reference.

• Gmail 554 5.6.0

  Email message is malformed. Not accepted. For more information, go to Email sender guidelines and review RFC 5322 specifications.

Common questions

What does 5.6.0 mean?

X.6.0 says something about the message content made it undeliverable, without a more specific code. Many providers use 5.6.0 for malformed messages.

What is the difference between 5.6.0 and 4.6.0?

They share the subject and detail digits, so they report the same condition, "Other or undefined media error". The first digit is the class: 4.x.x persistent transient failure: retrying later may succeed; 5.x.x permanent failure: resending unchanged will not succeed.

What should I do about 5.6.0?

Check the message's MIME structure and headers for errors. Sending a plain, well-formed test message is a quick way to confirm.

Would an email verifier have caught 5.6.0?

No. This reply is about your sending setup, the message or the receiving system, not about whether the address exists, so verifying the list will not prevent it.

Full guides for this code

• 554 5.6.0 — message malformed