Why is Outlook breaking DKIM keys and how can I fix it?
Michael Ko
Co-founder & CEO, Suped
Published 21 May 2025
Updated 16 Aug 2025
7 min read
Dealing with DKIM failures, especially when they primarily occur with Outlook or Hotmail, can be a frustrating experience. It’s perplexing when your emails pass authentication with other major mailbox providers like Gmail and Yahoo, but consistently encounter issues with Microsoft properties. This discrepancy often points to specific sensitivities within Outlook’s mail handling processes rather than a fundamental flaw in your DKIM setup.
DKIM, or DomainKeys Identified Mail, is a critical email authentication method designed to detect email spoofing and tampering. It works by adding a digital signature to outgoing emails, which recipient servers can then verify using a public key published in your domain’s DNS records. If any part of the signed email content is altered in transit, the signature breaks, leading to a DKIM failure.
While DKIM is designed to be robust, certain scenarios can cause it to fail, particularly with specific receiving mail servers. Outlook (including Office 365, Hotmail, and Outlook.com) has historically been known for its unique behaviors that can unintentionally invalidate DKIM signatures. This article delves into why this happens and what steps you can take to mitigate these issues and ensure your emails are properly authenticated.
One of the primary reasons Outlook tends to break DKIM signatures is its aggressive content modification policies. Microsoft’s email servers sometimes alter the email body or headers in ways that invalidate the original DKIM signature. This can happen for various reasons, such as adding footers, tracking pixels, or re-encoding characters, all of which change the message content after it has been signed.
For instance, if an email is signed with DKIM using a strict canonicalization algorithm, even minor changes to whitespace or character encoding can cause the body hash to mismatch, leading to a DKIM failure. Outlook’s systems might reformat HTML, introduce new line breaks, or convert certain characters to their ASCII equivalents, which are all actions that DKIM is designed to detect as tampering.
Forwarding is another common scenario where Outlook can break DKIM. When an email is forwarded through an Outlook.com or Office 365 account, the content can be altered significantly. This modification invalidates the original DKIM signature, resulting in a failed verification at the final recipient’s server, even if the initial email was perfectly authenticated.
Common reasons for DKIM breakage
Beyond Outlook’s specific behaviors, several general issues can lead to DKIM failures, which might be amplified when sending to Microsoft properties. One common problem is incorrect DNS configuration. This includes typos in your DKIM record, missing TXT records, or issues with your DNS provider not propagating changes correctly. Even subtle errors like extra spaces or tabs in the DNS record can invalidate the signature.
Another factor is the DKIM key length. While 1024-bit keys were once common, many providers, including Microsoft, now prefer or implicitly require 2048-bit DKIM keys for enhanced security. If your key is too short, or if the DNS record for a 2048-bit key is not properly split to accommodate DNS TXT record length limits, it can result in a DKIM failure or temperror status.
Encoding issues
Microsoft’s mail servers are particularly sensitive to character encoding. If your email contains non-ASCII characters or uses an encoding other than UTF-8, Outlook might re-encode the message, inadvertently breaking the DKIM signature. This is a common cause of DKIM body hash did not verify errors.
Strict DMARC policies
While not a direct cause of DKIM breaking, if your domain has a DMARC policy set to p=reject or p=quarantine, a broken DKIM signature could lead to emails being rejected or sent to spam, even if SPF passes. This is because DMARC requires at least one of SPF or DKIM to align to pass.
Diagnosing and troubleshooting Outlook DKIM failures
The first step in fixing DKIM issues with Outlook is to gather diagnostic information. If you have access to DMARC reports from Microsoft, these can provide valuable insights into why your DKIM signatures are failing. Look for specific failure reasons like 'body hash did not verify' or 'temperror'.
If DMARC reports aren't immediately available, you can perform a manual check. Send an email from your domain to an Outlook.com address and then examine the email headers. Look for the Authentication-Results header, which will indicate the DKIM status (pass, fail, softfail, temperror, or none) and often provide a reason for any failure.
You can also use an online email deliverability tester to get a quick overview of your DKIM setup. These tools can check your DNS records for common issues and provide recommendations. Additionally, verify your SPF record and DMARC policy, as these authentication methods work in conjunction with DKIM to build your domain's sending reputation and ensure proper deliverability.
Implementing solutions and best practices
To address DKIM breakage with Outlook, focus on minimizing any potential message modifications. Ensure your email sending platform or MTA uses relaxed canonicalization for both headers and body, if possible. This allows for slight modifications without invalidating the signature. Also, standardize your email encoding to UTF-8 to prevent Outlook’s servers from re-encoding content.
Regularly review and rotate your DKIM keys. Ensure you are using 2048-bit keys and that they are correctly published in your DNS. If your DNS provider has length restrictions for TXT records, you might need to split your DKIM record into multiple parts, a common workaround for longer keys. Regularly checking your domain’s blocklist (or blacklist) status can also reveal underlying reputation issues impacting deliverability.
Email Content: Avoid unnecessary formatting, hidden characters, or non-standard encoding that Outlook might alter. Stick to UTF-8 and clean HTML.
For ongoing issues, consider using dedicated email deliverability monitoring platforms. These services can provide detailed insights into how your emails are performing across different mailbox providers, helping you identify patterns of failure and pinpoint the exact cause of DKIM failures specifically with Outlook and other recipients.
Views from the trenches
Best practices
Always use UTF-8 encoding for your email content to ensure compatibility with Outlook's systems.
Implement relaxed canonicalization for DKIM signatures to allow for minor modifications by recipient servers.
Regularly rotate your DKIM keys and ensure they are at least 2048-bit for enhanced security.
Routinely monitor your DMARC reports to detect and diagnose DKIM failures and authentication issues.
Common pitfalls
Not accounting for Outlook’s content modification, which can inadvertently break DKIM signatures.
Using older 1024-bit DKIM keys that are considered less secure and may face stricter validation.
Failing to split long DKIM TXT records, leading to DNS lookup errors or truncated keys.
Ignoring character encoding issues, particularly with non-ASCII characters, which can cause body hash mismatches.
Expert tips
Verify that your email service provider or MTA supports relaxed canonicalization for optimal DKIM pass rates.
Consider testing emails to various Outlook.com addresses to identify specific formatting or content triggers.
Ensure DNS record changes for DKIM have fully propagated globally before assuming fixes are implemented.
Leverage DMARC aggregate reports to gain a comprehensive overview of DKIM authentication results from Microsoft.
Marketer view
Marketer from Email Geeks states that Outlook can break DKIM keys, even for direct sends, likely due to internal message modifications occurring before validation.
Nov 15, 2021 - Email Geeks
Marketer view
Marketer from Email Geeks pointed out that any alteration of the email by the receiving server will invalidate the DKIM signature, which is central to DKIM's purpose, and suggested ensuring character encoding like UTF-8.
Nov 15, 2021 - Email Geeks
Ensuring email authentication success with Outlook
While Outlook’s behavior can be challenging, understanding the common causes of DKIM breakage, both specific to Microsoft and general in nature, is key to resolving these issues. By meticulously checking your DNS records, ensuring proper email encoding, using relaxed canonicalization, and maintaining vigilance with DMARC reports, you can significantly improve your DKIM pass rates and overall email deliverability to Microsoft properties.
