MailFixIt

Generic SMTP delivery response

Understand SMTP 5.6.0: Message format failure

The receiving system could not accept the message content or media format but did not provide a more specific enhanced code.

Problem description

A representative response is:

550 5.6.0 Message format failure.
Is this temporary or permanent?

The same message content is likely to fail again. Correct or regenerate it before retrying.

What this response means

Standard 5.6.0 Other or undefined media error is a permanent content-class failure without a more precise subtype. The human-readable response and raw message source are essential.

Fix it

  1. Preserve the original raw message, full NDR, and sending application version.
  2. Look for the exact object named by the receiver: malformed MIME boundaries, invalid transfer encoding, unsupported content, broken headers, or an unsafe attachment.
  3. Rebuild the message with a maintained MIME library instead of editing encoded source by hand.
  4. Remove or replace only the content identified by policy; do not merely rename a prohibited attachment.
  5. Send a newly generated minimal test, then add content back until the faulty component is isolated.

DNS authentication cannot repair malformed MIME, although a receiver may report a separate authentication error afterward.

Who can fix it

The application or mail system that generated or transformed the message owns the repair.

Use a provider-specific page when its response names a particular header, attachment, or RFC violation.

Frequently asked questions

What does SMTP 5.6.0: Message format failure mean?

Something about the message content or media made it undeliverable, but the generic code does not identify the exact malformed or unsupported part.

Can MailFixIt prove this entire bounce from public DNS?

No. The current checker can evaluate selected public DNS configuration, but it cannot recreate recipient state, credentials, reputation, message content, or a historical provider SMTP decision.

Related email diagnostics

SMTP 550 5.7.26: Multiple authentication checks failedUnderstand SMTP 5.7.26 and separate SPF, DKIM, DMARC, and alignment failures before changing email DNS.

Primary sources: IANA SMTP Enhanced Status Codes.