Summary
The Yahoo bounce message '554 Message not allowed - Headers are not RFC compliant' arises from various issues violating email header formatting standards defined by RFC specifications. These include incorrect header formatting (missing colons, spaces, incorrect line endings), improper line wrapping (especially CRLF usage), DKIM signature wrapping problems, line breaks in unexpected places (e.g., after X-IncomingTopHeaderMarker), non-matching email addresses in From/Reply-to, missing or invalid Content-Type headers, incorrect character encoding (particularly with non-ASCII characters), header injection vulnerabilities, exceeding maximum line lengths without proper wrapping, multiple Reply-To headers, missing MIME version, and incorrectly formatted Date headers. Addressing these involves using email testing tools, sanitizing user inputs, implementing strict header validation, configuring proper email authentication (SPF, DKIM, DMARC), ensuring correct DKIM key size and setup, adhering to RFC 5322 guidelines, verifying single Reply-To headers, including the MIME-Version header, consulting with developers (especially for in-house MTAs), and potentially creating Exchange Online transport rules to bypass Clutter checks.
Key findings
- Header Formatting: Incorrect header formatting is a common cause, including missing colons/spaces, incorrect line endings.
- Line Wrapping: Improper line wrapping, especially related to CRLF usage and exceeding maximum line lengths, violates RFC standards.
- DKIM Issues: DKIM signature wrapping problems and incorrect key size/setup contribute to the error.
- Security Vulnerabilities: Header injection vulnerabilities introduce malicious headers, violating RFC standards.
- Encoding Problems: Incorrect character encoding, particularly with non-ASCII characters, creates compliance issues.
- Inconsistent Email Names: The Reply-to and From headers are not RFC compliant because the friendly name email address does not match the actual email address
- Multiple Reply-To: Including more than one Reply-To headers is against RFC standards and will cause bounces
- MIME-Version: The MIME-Version header needs to be included. It is against RFC standards not to include it
- Date Header: Incorrectly formatted Date: headers are a significant cause of bounces.
Key considerations
- Testing Tools: Use email testing tools to analyze raw headers for potential problems before sending.
- Input Sanitization: Sanitize user inputs to prevent header injection and ensure proper escaping of special characters.
- Header Validation: Implement strict header validation to enforce RFC compliance.
- Authentication: Configure SPF, DKIM, and DMARC to improve deliverability and reduce scrutiny of header content.
- Consult Developers: Engage developers, especially with in-house MTAs, to troubleshoot header-related issues.
- Review RFC 5322: Thoroughly review and implement RFC 5322 guidelines for email header formatting.
- Implement Line Wrapping: Ensure proper line wrapping according to RFC specifications.
- Exchange Rule: Configure a transport rule to bypass Clutter if using Exchange Online.
- Check DKIM Setup: Verify DKIM signature wrapping and ensure correct key size and configuration.
- Single Reply-To: Verify that only one Reply-To header is included in the email.
- MIME Version Included: Ensure MIME-Version header is set.
- Correct Date: Ensure Date headers is set correctly.
What email marketers say
12 marketer opinions
The '554 Message not allowed - Headers are not RFC compliant' bounce message from Yahoo indicates that the email headers do not adhere to the formatting standards defined by RFC specifications. Common causes include improper header formatting (missing colons/spaces, incorrect line endings), DKIM signature wrapping issues, line breaks in unexpected places, non-matching email addresses in the From/Reply-to headers, multiple Reply-to headers, missing MIME version, character encoding problems (especially with non-ASCII characters), header injection vulnerabilities, and exceeding maximum line lengths without proper wrapping. Utilizing email testing tools, sanitizing user inputs, implementing correct header validation, and configuring proper email authentication can help resolve these issues.
Key opinions
- Header Formatting: Incorrect header formatting (missing colons/spaces, incorrect line endings) is a common cause.
- DKIM Wrapping: DKIM signature lines not wrapping correctly can trigger the error.
- Header Injection: CRLF injection vulnerabilities can introduce malicious headers, violating RFC standards.
- Encoding Issues: Improper character encoding, particularly with non-ASCII characters, can lead to compliance problems.
- Line Length: Exceeding maximum header line lengths (typically 78 characters) without proper wrapping causes issues.
- Inconsistent email names: The Reply-to and From headers are not RFC compliant because the friendly name email address does not match the actual email address
- Multiple Reply-To's: Including more than one Reply-To headers is against RFC standards and will cause bounces
- MIME-Version: The MIME-Version header needs to be included. It is against RFC standards not to include it
Key considerations
- Email Testing Tools: Utilize email testing tools to analyze raw headers before sending to identify potential problems.
- Input Sanitization: Sanitize user inputs to prevent header injection attacks and ensure proper escaping of special characters.
- Header Validation: Implement strict header validation to enforce RFC compliance and prevent unauthorized modifications.
- Authentication: Configure SPF, DKIM, and DMARC to improve deliverability and reduce scrutiny of header content.
- Update Email Addresses: Update email addresses to be valid format.
Marketer view
Email marketer from SuperUser points out that ensuring proper encoding of characters in headers, especially non-ASCII characters, is vital for RFC compliance. Using appropriate encoding schemes like UTF-8 and correctly implementing MIME encoding can resolve encoding-related header issues.
22 Dec 2021 - SuperUser
Marketer view
Email marketer from Email Geeks explains that the `Reply-to` and `From` headers are not RFC compliant because the friendly name email address does not match the actual email address. Also points out that two `reply-to` headers will cause a reject.
5 Mar 2025 - Email Geeks
What the experts say
5 expert opinions
The '554 Message not allowed - Headers are not RFC compliant' bounce message from Yahoo can be resolved by addressing several potential issues within the email headers. Experts recommend a thorough check for non-RFC compliant elements, with a focus on proper line wrapping, especially CRLF usage. Security vulnerabilities like header injection must be mitigated through input sanitization and strict header validation. Ensuring the Date: header adheres to RFC 5322 formatting is critical. The email should contain only a single Reply-To header and must include the MIME-Version header to comply with RFC standards. Consulting developers, especially when using in-house MTAs, is advised for troubleshooting these complex issues.
Key opinions
- Line Wrapping: Improper line wrapping in headers, particularly related to CRLF usage, is a common cause of RFC compliance errors.
- Header Injection: Header injection vulnerabilities can lead to the insertion of malicious headers, violating RFC standards.
- Date Formatting: Incorrectly formatted Date: headers, not adhering to RFC 5322, are a significant cause of bounces.
- Reply-To Header: Including more than one Reply-To header violates RFC standards and results in bounce messages.
- MIME-Version Header: The MIME-Version header must be present to comply with RFC standards.
Key considerations
- Consult Developers: Engage developers, particularly with in-house MTAs, to assist in troubleshooting header-related issues.
- Input Sanitization: Sanitize user input to prevent header injection attacks and ensure proper escaping of special characters.
- Header Validation: Implement strict header validation to enforce RFC compliance and prevent unauthorized modifications.
- RFC 5322 Adherence: Ensure all date formats comply with RFC 5322 specifications.
- Single Reply-To: Verify that only one Reply-To header is included in the email.
- MIME Version Included: Ensure MIME-Version header is set.
Expert view
Expert from Word to the Wise advises to only include a single Reply-To header. Including more than one is against RFC standards and will cause bounces.
17 Mar 2022 - Word to the Wise
Expert view
Expert from Email Geeks advises to check the headers for non-RFC compliant elements, guessing it might be a line that's not wrapped properly. Suggests consulting with the developers to troubleshoot, especially if using an in-house MTA and suggests the problem might be a LF when there should be a CRLF.
29 Dec 2024 - Email Geeks
What the documentation says
4 technical articles
To resolve the '554 Message not allowed - Headers are not RFC compliant' error from Yahoo, it is critical to adhere to RFC 5322, which defines the Internet Message Format and mandates specific formatting for email headers, including CRLF for separating header fields. Long header lines must be wrapped per RFC specifications, using a CRLF followed by whitespace. For Exchange Online users, creating a transport rule to bypass Clutter checks can prevent filtering issues related to RFC non-compliance. Additionally, DKIM signatures should be checked and corrected for proper wrapping, and DKIM keys should be appropriately sized (at least 1024 bits) and correctly configured.
Key findings
- RFC 5322 Compliance: Adhering to RFC 5322 for header formatting is crucial, including proper CRLF usage.
- Line Wrapping: Long header lines require proper wrapping with CRLF and whitespace to avoid compliance errors.
- Exchange Online Clutter: Exchange Online users may need a transport rule to bypass Clutter checks that can flag RFC non-compliant messages.
- DKIM Signature: DKIM signature lines can cause issues if they are not wrapped correctly, and DKIM keys must be properly configured.
Key considerations
- Review RFC 5322: Thoroughly review and implement RFC 5322 guidelines for email header formatting.
- Implement Line Wrapping: Ensure all header lines are properly wrapped according to RFC specifications.
- Configure Exchange Rule: If using Exchange Online, create a transport rule to bypass Clutter if RFC compliance issues persist.
- Check DKIM Setup: Verify DKIM signature wrapping and ensure that DKIM keys are correctly sized and configured.
Technical article
Documentation from DKIM explains about DKIM-Signature line wrapping issues. Check the DKIM signature and fix the signature. Ensure that your key is set up correctly. Using a key size of at least 1024 bits is recommended.
11 May 2024 - DKIM
Technical article
Documentation from Postfix.org explains that long header lines must be wrapped according to RFC specifications. This involves inserting a CRLF followed by a whitespace character before exceeding the maximum line length. Improperly wrapped header lines can trigger RFC compliance errors.
3 Aug 2023 - Postfix.org
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