How do I fix Yahoo bounce message '554 Message not allowed - Headers are not RFC compliant'?

Key findings

• Header non-compliance: The error directly points to issues with the email's header formatting, which can vary from incorrect line endings (e.g., LF instead of CRLF) to malformed or missing header fields. This is a common cause of Yahoo and AOL bounces.
• Specific header problems: Common culprits include improper wrapping of the DKIM-Signature line, issues with the 'From' and 'Reply-to' headers (e.g., email addresses in the friendly name portion that do not match the actual address, or multiple 'Reply-to' headers), or missing standard headers like 'Message-ID' or 'Date'. Stack Overflow users often highlight missing essential headers.
• Impact of canonical rewriting: When using Postfix with sender canonical rewriting, misconfigurations can inadvertently create non-compliant headers, even if the intention is to properly sign emails with DKIM or align with DMARC.
• Server-side versus application-side: The issue often originates from how the email is generated by the application or mail transfer agent (MTA) before it even reaches the final recipient. Therefore, troubleshooting may require examining the raw headers generated by your sending system.

Key considerations

• Inspect raw headers: Always examine the raw email headers of bounced messages. This is the first step in diagnosing RFC compliance issues, as it reveals the exact format being sent.
• Header formatting: Ensure proper line wrapping (CRLF) for all headers, especially DKIM-Signature. Headers exceeding 998 characters should be 'folded' onto multiple lines. This is crucial for overall email deliverability.
• From and Reply-to integrity: Verify that your 'From' and 'Reply-to' headers are correctly formatted. If using friendly names, ensure they are enclosed in double quotes. Avoid placing email addresses within the friendly name part, or at least ensure they perfectly match the actual email address.
• Unique headers: Ensure only one instance of headers like 'Reply-to' is present in the email, as duplicate headers can cause rejection. Also, confirm the presence of essential headers such as 'Message-ID' and 'Date'.
• Application level fixes: Changes to resolve RFC non-compliance often need to be made at the application code level, rather than solely relying on MTA configurations. Regularly troubleshoot email bounce messages.

Key opinions

• Header inspection: Marketers frequently stress the need to examine the raw email headers to pinpoint what specifically is causing the non-compliance error. It's often not immediately obvious from the bounce message itself.
• Line wrapping: A common opinion is that incorrect line wrapping, such as using a Line Feed (LF) instead of a Carriage Return Line Feed (CRLF) for header breaks, is a frequent cause of RFC non-compliance.
• MTA configuration: Marketers and developers using in-house MTAs like Postfix often find that the issue lies within how their MTA or its associated components (like OpenDKIM/OpenDMARC) are configured to generate or modify headers.
• From/Reply-to issues: Issues with 'From' and 'Reply-to' headers are frequently cited. This includes non-matching email addresses between the friendly name and the actual address, or malformed entries.
• Sender canonical rewrites: Some marketers use sender canonical rewrite rules (e.g., in Postfix) for DKIM signing, but these can unintentionally introduce RFC compliance problems if not implemented carefully.

Key considerations

• Developer involvement: Marketers often advise collaborating with developers to troubleshoot and fix these specific header issues, as they typically require changes to the sending application or mail server configuration.
• Careful header construction: When generating email headers, especially 'From' and 'Reply-to', ensure that the friendly part is properly quoted and that any email addresses match perfectly. For more information, see what RFC 5322 says versus what works.
• Avoid duplicate headers: One key lesson is to prevent the generation of duplicate headers, such as two 'Reply-to' headers, as this is a clear violation of RFC standards and will result in bounces.
• Test thoroughly: After making any changes to header generation or MTA configuration, thorough testing to various mailbox providers (like Gmail, Outlook, Yahoo) is crucial. A bounce from Yahoo might indicate similar issues with other providers, or AOL might also reject emails.
• Review Postfix configuration: For Postfix users, reviewing 'sender_canonical' files and other configurations that might alter email headers is essential to ensure they do not introduce RFC non-compliance.

Key opinions

• Header non-compliance: Experts agree that specific header fields like 'From' and 'Reply-to' are frequent sources of RFC compliance errors, particularly when friendly names are improperly formatted or mismatched.
• Reputational impact: Sending emails with non-compliant headers, especially with mismatched addresses in the friendly name, mimics behavior seen from 'bad actors' and can negatively impact sender reputation and inbox placement.
• DKIM vs. headers: While DKIM is crucial for domain authentication, experts clarify that it doesn't absolve the need for individual header compliance. A valid DKIM signature won't fix a malformed 'From' header.
• Duplicate headers: The presence of duplicate headers, such as two 'Reply-to' fields, is a clear RFC violation and a guaranteed cause of email rejection by recipient servers.
• Importance of raw headers: Experts strongly recommend providing and analyzing raw email headers of rejected messages, as this provides the precise data needed for accurate diagnosis.

Key considerations

• Friendly name formatting: When using friendly names in 'From' or 'Reply-to' headers, ensure they are properly enclosed in double quotes. Ideally, avoid putting email addresses in the friendly part, or at least ensure they perfectly match the actual email address.
• Consistency is key: Ensure consistency between the display name and the email address in headers to avoid triggering spam filters. This is particularly important for your email domain reputation.
• Application-level fixes: Many RFC compliance issues stem from the application generating the email. Fixes should be implemented at this level to ensure proper header construction from the source.
• Monitoring DMARC reports: While not directly about RFC compliance, DMARC reports from Google and Yahoo can sometimes indirectly reveal underlying header issues if they lead to authentication failures.
• Thorough header analysis: A detailed analysis of all headers, beyond just the obvious ones, can uncover subtle non-compliance issues. Pay attention to issues like line breaks after blank header fields.

Key findings

• RFC 5322 mandate: RFC 5322 is the primary specification for email message format, including headers. It dictates precise rules for syntax, characters, and structure, and non-compliance will lead to rejections from major mailbox providers like Yahoo and Verizon Media Group.
• Header field structure: Documentation specifies that each header field consists of a field name, a colon, and a field body. The field name identifies the type of information, while the field body contains the actual data. A '554 Message not allowed' error is often a direct result of improper structure.
• Folding long headers: RFCs define a mechanism called 'folding' for long header fields. This involves breaking lines with a CRLF followed by one or more spaces or tabs, allowing for readability while maintaining compliance.
• Mandatory headers: Certain headers, such as 'Date' and 'Message-ID', are explicitly or implicitly required for proper email functionality and RFC compliance. Their absence can lead to rejection.
• Address specification: RFCs detail the syntax for email addresses, including the use of angle brackets for the address itself and optional double quotes for friendly names. Deviations here are common sources of errors.

Key considerations

• CRLF endings: All line breaks within email headers must use CRLF (carriage return followed by line feed), not just LF. This is a fundamental RFC requirement.
• Unique headers: For specific header fields like 'From', 'Subject', and 'Message-ID', only one instance should appear in the email, as multiple occurrences are generally non-compliant and can cause bounce messages.
• Address integrity: The email address specified in the 'From' header must be valid and resolvable, ensuring that the domain exists and can receive bounces or replies.
• Programmatic implementation: When building custom email sending solutions, ensure that libraries or code explicitly construct headers in accordance with RFC 5322. Relying solely on default settings may not always be sufficient. This is a crucial step in email authentication.