Why is my DKIM body hash failing and how do I fix it?
Michael Ko
Co-founder & CEO, Suped
Published 8 Jul 2025
Updated 15 Aug 2025
7 min read
Seeing a "DKIM body hash did not verify" error can be incredibly frustrating. It means that while your email might appear to have a DKIM signature, the content of the message has been altered after it was signed. This alteration, even something as minor as a single space or line break, causes the receiving mail server to calculate a different hash value than the one recorded in your DKIM signature, leading to an authentication failure.
This isn't just a minor technical glitch. A failing DKIM body hash can severely impact your email deliverability, often causing your messages to land in recipients' spam folders or be rejected outright. Understanding why this happens and how to address it is crucial for maintaining a healthy sending reputation and ensuring your emails reach their intended inboxes.
DKIM (DomainKeys Identified Mail) works by allowing an organization to cryptographically sign outgoing emails. This signature is then verified by the recipient's mail server using a public key published in your domain's DNS records. Part of this signature includes a body hash, which is a unique fingerprint of your email's content. It's designed to ensure the message body remains untouched from the moment it's signed until it reaches the recipient. If even a small change occurs, the calculated hash will no longer match the signed hash, and DKIM authentication will fail.
The way this hash is calculated is influenced by what's called canonicalization. There are two primary types: simple and relaxed. Simple canonicalization is very strict, meaning any modification to whitespace or line endings will cause the hash to fail. Relaxed canonicalization is more forgiving, allowing for minor changes in whitespace and blank lines without invalidating the signature. The choice of canonicalization for both the header and the body of the email plays a significant role in how susceptible your DKIM signature is to post-signing modifications.
When a DKIM body hash fails, it tells the receiving server that the email's integrity has been compromised. This triggers a red flag, as it suggests the email might have been tampered with or is part of a spoofing attempt. Consequently, your email's chances of reaching the inbox diminish significantly, potentially leading to it being quarantined or marked as spam.
Common causes of DKIM body hash failures
The primary reason for a DKIM body hash failure is that the email body has been altered after the original DKIM signature was applied by your sending server. This can happen at various points in the email's journey, often by legitimate intermediaries.
Email gateways and security services
Many organizations use email security services, such as Microsoft Defender for Office 365, Mimecast, or Proofpoint. These services often add disclaimers, footers, track links (URL rewriting), or make other minor formatting adjustments to the email body. If these modifications occur after your email server has applied the DKIM signature, the body hash will no longer match. This is a common cause, as documented by various email service providers.
Mailing lists and forwarders
When an email is sent through a mailing list or forwarded by a recipient's mail server, the content is frequently modified. These systems might add headers, footers, or even reformat the message body in subtle ways, such as altering line breaks or encoding. Such changes will break the original DKIM signature's body hash, causing authentication to fail at the final destination.
Character encoding and line ending issues
Inconsistencies in how email content is encoded or how line endings (CRLF vs. LF) are handled can also lead to body hash failures. Different operating systems or mail transfer agents (MTAs) might interpret and apply these differently. For instance, a Byte Order Mark (BOM) in a UTF-8 file can introduce invisible characters that alter the body hash. As explained by Broadcom, any change to the message body, including additions or removals, will cause the body hash validation to fail.
Diagnosing the problem
The first step in diagnosing a DKIM body hash failure is to retrieve the email headers of a problematic message. You'll typically find a line that starts with "Authentication-Results" or "X-DKIM-Status" indicating the DKIM verification result. Look for specific messages like dkim=fail (body hash did not verify) or bodyhash_mismatch.
Using a reliable email deliverability tester can provide a detailed breakdown of your email's authentication status, including specific errors related to DKIM. This helps confirm whether the issue is consistently present across different recipients or only with specific mailbox providers like Microsoft Outlook. Understanding the exact error message and the path your email takes can help pinpoint where the modification is occurring.
Scenario: DKIM pass
Authentication Results: dkim=pass (signature was verified)
Email Body: No modifications after signing, preserving original content.
Deliverability Impact: Negative, increased risk of spam folder or rejection.
Implementing the fixes
One of the most effective solutions is to adjust your DKIM canonicalization settings. If your DKIM record is currently set to "simple" canonicalization for the body, consider changing it to "relaxed." This makes the hash calculation more tolerant of minor changes like whitespace or header folding, which are common when emails pass through various intermediaries. This change can often resolve body hash failures caused by benign modifications.
Example DKIM TXT record with relaxed canonicalization
If you're using an email security gateway or a service that processes your outgoing mail, investigate its configuration. Many services allow you to configure them to apply their modifications before your email server signs the message. Alternatively, some services can be configured to not modify the signed parts of the email. This is particularly relevant for Outlook.com and Microsoft 365 users, where third-party services can interfere with DKIM. Also ensure your email content generation is consistent. Avoid hidden characters, ensure uniform line endings (CRLF), and use a consistent character set (e.g., UTF-8) to prevent unexpected changes to the body that could lead to a DKIM body hash mismatch.
The impact of DKIM body hash failure
A failing DKIM body hash means your emails are not fully authenticated, leading to a diminished sender reputation. This often results in messages being marked as spam or rejected, affecting your overall email deliverability. Since DMARC policies often rely on DKIM authentication, a body hash failure can also lead to DMARC failure, further impacting your email's journey to the inbox. It's crucial to address these issues promptly to ensure your email program functions effectively.
Views from the trenches
Best practices
Ensure DKIM canonicalization for both header and body is set to 'relaxed' if your emails pass through intermediaries that might subtly alter content.
Prioritize configuring email security services to apply modifications before DKIM signing to prevent body hash mismatches.
Regularly monitor your email authentication results using a trusted deliverability tool to catch body hash failures early.
Common pitfalls
Overlooking subtle changes introduced by mailing lists or forwarding services, which can break the DKIM body hash.
Using 'simple' canonicalization when your email flow includes systems that modify email content, leading to frequent DKIM failures.
Failing to standardize character encoding and line endings in your email sending process across different systems.
Expert tips
Always inspect raw email headers to identify the exact DKIM failure message; it provides critical clues for troubleshooting.
Be aware that third-party email security solutions are frequent culprits for modifying email bodies after signing, which breaks DKIM.
Remember that even seemingly minor changes, like adding or removing a single space, can cause a body hash to fail validation.
Marketer view
Marketer from Email Geeks says that if the body hash fails to verify, it indicates that the hash generation is incorrect, and DKIM will not pass.
2020-07-22 - Email Geeks
Marketer view
Marketer from Email Geeks says that a body hash failure suggests either incorrect signing or message modification during transit, which is a serious issue if DKIM authentication is important for your messages.
2020-07-22 - Email Geeks
Restoring trust in your emails
Resolving DKIM body hash failures is essential for maintaining strong email deliverability and sender reputation. These failures indicate that your email's content is being altered after it's signed, compromising its authenticity. By understanding the common causes, carefully diagnosing the specific points of modification, and implementing the appropriate fixes such as adjusting canonicalization or reconfiguring intermediary services, you can ensure your emails are properly authenticated and consistently reach their intended recipients.
Proactive monitoring and continuous testing are key to preventing these issues from recurring. Regular checks of your email headers and using deliverability tools will help you stay ahead of potential problems, safeguarding your email program from unexpected authentication failures.
