Yahoo and AOL message-format error

Fix Yahoo RFC header compliance failures

Yahoo groups duplicate headers, incorrectly formatted headers, incorrect MIME types, and similar defects as RFC compliance failures. Repair the message generator or rewriting gateway—not the domain's authentication DNS.

Problem description

A representative response is:

554 Message not accepted: headers are not RFC compliant

Diagnostic availability: MailFixIt accepts selected headers as context but does not validate complete RFC 5322 and MIME message bodies. This page has no checker CTA.

Is this temporary or permanent?

A Yahoo 554 response is permanent. Generate a structurally corrected message before resubmission.

What to inspect

Missing, duplicated, or malformed From, Date, Message-ID, or address headers.
Invalid line endings or header folding.
Broken MIME Content-Type, boundaries, charsets, or encodings.
A gateway that modifies raw content incorrectly.

Resolution sequence

Capture raw output before SMTP submission.
Validate it with a standards-aware parser.
Upgrade or configure the mail library rather than concatenating raw fields.
Compare content before and after each relay.
Send a minimal valid message, then restore features incrementally.

Frequently asked questions

What does a Yahoo RFC compliance failure mean?

Yahoo found a malformed header, MIME type, or other message-format condition that does not meet Internet message standards.

Can SPF repair malformed headers?

No. The application or gateway generating or rewriting the message must be corrected.

What should I validate?

Validate the raw message's required and singleton headers, address syntax, line folding, MIME types, boundaries, and transfer encodings.

Related email diagnostics

Primary sources: Yahoo Sender Hub SMTP Error Codes, RFC 5321 SMTP, and RFC 5322 Internet Message Format.