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

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.

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.

Email marketer from Email on Acid explains that common header mistakes include missing or invalid Content-Type headers, incorrect character encoding declarations, and the presence of illegal characters. Reviewing and correcting these aspects can help ensure RFC compliance.

Email marketer from Gmass says that ensuring the From header contains a valid email address in the correct format is crucial. A malformed or missing From header can lead to RFC compliance issues and delivery problems.

Email marketer from AuthSMTP advises to configure proper email authentication (SPF, DKIM, DMARC) as it can sometimes indirectly help with RFC compliance issues. Although authentication primarily addresses sender verification, it can reduce the likelihood of email servers scrutinizing headers for potential spam indicators, which might include RFC violations.

Email marketer from Reddit user shares that CRLF injection vulnerabilities can lead to the insertion of malicious headers, causing RFC compliance failures. Sanitizing user input and properly escaping special characters in headers is crucial to prevent such issues.

Email marketer from cPanel Forum mentions that removing or encoding invalid characters in email headers is necessary for RFC compliance. Special characters or control characters that are not properly encoded can cause parsing errors and trigger bounce messages.

Marketer from Email Geeks guesses that the DKIM-Signature line isn't wrapping properly, or rather, not wrapping properly.

Email marketer from Litmus shares that using email testing tools to analyze the raw email headers before sending can help identify any RFC compliance issues. These tools can highlight formatting errors, invalid characters, or other discrepancies that may cause bounce messages.

Email marketer from Stack Overflow user explains that the issue might stem from incorrect header formatting, such as missing colons or spaces after header names, or incorrect line endings. Correcting these formatting errors in the email headers can resolve the RFC compliance issue.

Marketer from Email Geeks points out a linebreak after "X-IncomingTopHeaderMarker:" in the Hotmail headers.

Email marketer from MailChannels explains that ensuring all header lines are below the maximum allowed length (typically 78 characters) is crucial. Long header lines must be folded (wrapped) according to RFC specifications to avoid compliance errors.

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.

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.

Expert from Spam Resource says to check Date: header is properly formatted according to RFC 5322. Incorrectly formatted dates are a very common cause for bounces.

Expert from Spam Resource highlights that header injection vulnerabilities, where attackers insert malicious headers, can lead to RFC non-compliance. The fix involves sanitizing user input and implementing strict header validation to prevent unauthorized modifications.

Expert from Word to the Wise states that the MIME-Version header needs to be included. It is against RFC standards not to include it.

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.

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.

Documentation from Microsoft explains to create an Exchange Online transport rule to add an X-MS-Exchange-Organization-BypassClutter check to the message header. This can prevent filtering issues due to RFC non-compliance, especially when Microsoft's Clutter feature is enabled.

Documentation from RFC 5322 specifies the Internet Message Format, defining the syntax for email headers. It mandates specific formatting for headers, including the use of CRLF (carriage return line feed) to separate header fields and the proper encoding of characters. Ensuring adherence to RFC 5322 is crucial for avoiding the 'Headers are not RFC compliant' error.