How do I fix Yahoo bounce message '554 Message not allowed - Headers are not RFC compliant'?
Michael Ko
Co-founder & CEO, Suped
Published 15 Apr 2025
Updated 19 Aug 2025
9 min read
Encountering a '554 Message not allowed - Headers are not RFC compliant' bounce message from Yahoo can be quite frustrating. This error indicates that Yahoo's (or AOL's) mail servers have detected an issue with the formatting of your email's headers, preventing the message from being delivered. While other mailbox providers like Gmail or Hotmail might accept these emails, Yahoo often has stricter compliance checks.
The core of the problem lies in the email's metadata, specifically how the headers conform to the Request for Comments (RFC) standards, such as RFC 5322 which defines the format of internet messages. If your email headers deviate from these established guidelines, Yahoo will likely reject them.
Troubleshooting this issue requires a close look at your email's internal structure, particularly the headers. Since successful delivery to other providers doesn't guarantee Yahoo compliance, a focused investigation into your sending practices is essential. I'll guide you through identifying and resolving these common header-related issues to improve your email deliverability with Yahoo.
This problem is distinct from general bounce messages caused by spam filters or blocklists, as it points directly to a technical formatting error. For a broader overview of how to troubleshoot bounce messages, you can refer to our guide on how to troubleshoot email bounce messages.
What is RFC compliance in email headers?
Email headers are a set of metadata fields at the beginning of an email message, containing information about the sender, recipient, subject, and various routing details. These headers must strictly adhere to the standards outlined in various Request for Comments (RFC) documents published by the Internet Engineering Task Force (IETF).
When Yahoo (or Verizon Media Group properties like AOL, SBCGlobal, Rocketmail) returns a '554 Message not allowed - Headers are not RFC compliant' error, it means their mail servers found a violation in your message's header formatting. This could be anything from incorrect line breaks, invalid characters, malformed email addresses in the From or Reply-To fields, or even duplicate header fields that the RFCs explicitly forbid. This specific 554 bounce from Yahoo indicates issues with message format compliance, as detailed on the Yahoo Sender Hub.
RFC compliance is critical because it ensures interoperability across different email systems. Mailbox providers like Yahoo rely on these standards to parse, route, and deliver messages correctly. Deviations can be flagged as suspicious behavior, often associated with spam or phishing attempts, leading to immediate rejection.
Importance of RFC compliance
RFCs ensure that email messages are structured uniformly, allowing different email systems to understand and process them correctly. Without these rules, email communication would be chaotic and unreliable.
Common RFC violations
Violations often include improperly formatted addresses, incorrect header field syntax, excessively long lines, or the presence of duplicate headers that should only appear once. Word to the Wise provides additional insights into such issues.
Pinpointing common header pitfalls
Based on common issues, and the specifics of the bounce message, there are a few key areas where your email headers might be falling short of Yahoo's expectations. These often stem from how your email sending system (like Postfix) or application code (like smtplib) constructs the message.
One common culprit is the DKIM-Signature line not wrapping correctly. RFC standards dictate maximum line lengths for headers, and if a long header, like a DKIM signature, isn't properly folded onto multiple lines using appropriate indentation (CRLF + SP or CRLF + HTAB), it can lead to rejection. This is a subtle but significant detail that many automated systems can miss. You can learn more about DKIM with our simple guide to DMARC, SPF, and DKIM.
Another frequent issue is the presence of multiple Reply-To headers. RFC 5322 explicitly states that there should only be one Reply-To header in an email. If your application or MTA (Mail Transfer Agent) is inadvertently adding a second one, Yahoo will interpret this as non-compliant. This is a definite red flag for spam filters.
Finally, pay close attention to the formatting of your From and Reply-To headers, especially regarding the 'friendly name' part. If the friendly name contains an email address that doesn't match the actual email address in the angle brackets, or if it's not properly quoted, it can appear suspicious. This is a tactic often used by malicious senders, and Yahoo's systems are designed to detect it. Our article What RFC 5322 Says vs. What Actually Works covers similar nuanced RFC interpretations.
Common header compliance issues
DKIM line wrapping: The DKIM-Signature header is too long and not properly folded according to RFC 5322, potentially leading to errors.
Duplicate Reply-To: The email contains more than one Reply-To header, which is a violation of RFC standards.
Malformed From/Reply-To: The friendly name in these headers is incorrectly formatted, often containing an unquoted or mismatched email address.
Incorrect character encoding: Using non-standard characters in headers without proper encoding can trigger rejections.
Diagnosing and resolving header issues
The first step to fixing these issues is to get a copy of the full email headers from a message that bounced or was successfully delivered to another provider like Microsoft (Hotmail/Outlook) or Gmail. This will allow you to inspect the raw headers for any anomalies.
Once you have the headers, look for the following: If your DKIM-Signature appears as one very long line without breaks, this is likely your issue. You'll need to ensure your mail software (e.g., Postfix, Sendmail) or application code automatically folds these long lines. If your application is manually constructing headers, ensure you're inserting CRLF (carriage return-line feed) followed by a space or tab to break long lines.
Next, search for multiple instances of the Reply-To header. If you find more than one, your email sending logic is generating redundant headers. You'll need to trace back to the application code or MTA configuration (like sender_canonical maps in Postfix) to identify where the duplicate is being added and remove it. Yahoo is quite strict about this particular RFC violation, along with other 554 errors from Verizon Media Group.
Finally, examine your From and Reply-To headers for mismatched friendly names. Ideally, don't put an email address in the friendly part. If you do, ensure it precisely matches the actual email address. For example, instead of "name@example.com" <actual@example.com>, use "Your Name" <actual@example.com>. If an email address is included in the friendly name, it must be enclosed in double quotes.
Friendly name mismatch: The email address in the friendly name (e.g., noreply@equair.com.ec) does not match the actual email address (e.g., no-reply@kiusys.com). This looks suspicious.
Unnecessary email in friendly name: Putting an email address in the friendly name is generally discouraged unless it is an exact match and properly quoted.
Compliant from/reply-to headers
Example of compliant headerstext
Reply-to: "Support Team" <support@yourdomain.com>
From: "Your Company Name" <noreply@yourdomain.com>
Clear friendly names: Use descriptive names (e.g., "Your Company Name") that align with your brand, not email addresses.
Matching addresses (if applicable): If an email address is used in the friendly name, it must be the exact same as the actual email address and enclosed in quotes.
Once you've made these adjustments, send a test email to a Yahoo address and observe the bounce messages. Continuous monitoring of your email deliverability is key, especially with providers like Google and Yahoo tightening their sending requirements. You can learn about how these changes affect deliverability in our article on why AOL and Yahoo emails are bouncing.
Broader deliverability considerations
While fixing header compliance issues is crucial, it's part of a larger picture of email deliverability. Mailbox providers like Yahoo consider many factors when deciding whether to accept, reject, or place an email in the inbox or spam folder. Your sender reputation, email authentication, and content quality all play significant roles.
Ensure your SPF, DKIM, and DMARC records are correctly configured and aligned. These authentication protocols verify that your emails are legitimately from your domain and haven't been tampered with. A strong authentication setup is a fundamental requirement for good deliverability and helps prevent your domain from being added to a blocklist (or blacklist). For more information, see our guide to understanding DMARC reports.
Maintain a healthy sender reputation by sending relevant content to engaged recipients and avoiding spam complaints. High bounce rates, especially due to RFC compliance or other errors, can negatively impact your reputation and lead to your IP or domain appearing on email blocklists (or blacklists). You can learn about how these work from our simple guide to how email blacklists actually work.
Best practices for email deliverability
Implement DMARC: Ensure your DMARC policy is set to at least p=quarantine or p=reject for maximum protection and deliverability.
Monitor blocklists: Regularly check if your IP or domain is listed on any major email blocklists (or blacklists), as this can severely impact delivery. Our guide on what happens when your domain is on a blacklist explains the impact.
Engage your audience: Send valuable, relevant content to an engaged audience to maintain a positive sender reputation. This helps ensure your emails are placed in the inbox.
Clean your lists: Regularly remove inactive or invalid email addresses to reduce bounce rates and avoid spam traps.
Views from the trenches
Best practices
Always inspect full email headers, especially after making changes to your sending system or application code.
Ensure that your email sending library or MTA automatically handles header line wrapping for long fields like DKIM signatures.
Consistently review your DMARC reports for insights into authentication failures, which can often hint at header issues or spoofing attempts.
Common pitfalls
Ignoring duplicate header fields: RFCs are strict, and multiple instances of a field that should be unique will cause rejections.
Using email addresses in the friendly name part of 'From' or 'Reply-To' headers without proper quoting or matching the actual address.
Failing to update or verify email sending configurations after system migrations or software updates.
Expert tips
Use an email testing tool to send a test email and examine its raw headers for any RFC compliance issues before sending to live recipients.
Consult the documentation for your specific MTA (e.g., Postfix, Sendmail) or email library (e.g., smtplib) to ensure it's configured for RFC-compliant header generation.
For persistent issues, consider reaching out to your email service provider's support team or an email deliverability consultant for deeper analysis of your email flow.
Expert view
Expert from Email Geeks says to examine the headers to identify what is not RFC compliant, suspecting a line that is not wrapped properly.
2022-04-19 - Email Geeks
Expert view
Expert from Email Geeks says that a DKIM-Signature line not wrapping properly is a common cause for such bounce messages.
2022-04-19 - Email Geeks
Achieving Yahoo deliverability success
The '554 Message not allowed - Headers are not RFC compliant' bounce message from Yahoo is a clear indication that your email headers are not meeting the strict formatting standards. By meticulously inspecting your email headers for issues like improper line wrapping, duplicate header fields, or malformed From and Reply-To addresses, you can directly address the root cause of the rejection.
Remember, adhering to RFC standards isn't just about avoiding bounce messages, it's about building trust with mailbox providers and ensuring your messages are delivered reliably. Combining proper header formatting with robust email authentication (SPF, DKIM, DMARC) and strong sending practices will significantly improve your overall email deliverability to Yahoo and beyond.
Yahoo can be quite frustrating. This error indicates that Yahoo's (or 
AOL's) mail servers have detected an issue with the formatting of your email's headers, preventing the message from being delivered. While other mailbox providers like Gmail or Hotmail might accept these emails, Yahoo often has stricter compliance checks.
Microsoft (Hotmail/Outlook) or 
Gmail. This will allow you to inspect the raw headers for any anomalies.
Google and Yahoo tightening their sending requirements. You can learn about how these changes affect deliverability in our article on why AOL and Yahoo emails are bouncing.
