Why is DKIM failing in Hotmail but passing in Gmail?
Matthew Whittaker
Co-founder & CTO, Suped
Published 2 May 2025
Updated 16 Aug 2025
8 min read
When sending emails, especially transactional ones like flight information, it is perplexing to see a DKIM authentication fail with a provider like Hotmail (or Outlook.com) while the same email passes without issue when sent to Gmail. This inconsistency can lead to frustrating deliverability problems, often resulting in important messages landing in spam folders.
It is a common scenario that many senders encounter, highlighting the differing approaches email service providers take in verifying email authenticity.
DKIM (DomainKeys Identified Mail) is a crucial email authentication method designed to detect email spoofing. It allows the receiver to check if an email that claims to come from a specific domain was indeed authorized by the owner of that domain. This is done by attaching a digital signature to the email, which is then verified against a public key published in the sender's DNS records.
The core idea is simple: if the signature matches, the email's integrity has been maintained during transit, and it likely originates from a legitimate source. If it fails, it suggests the email might have been tampered with or forged. However, the interpretation and strictness of this verification can vary significantly between different email providers.
While Gmail typically focuses on the validity of the signature and its alignment with the sending domain, Microsoft (Hotmail and Outlook.com) tends to apply more stringent checks. These stricter validation rules can cause perfectly valid emails to fail DKIM, even when their setup seems correct and other major providers accept them.
Why Hotmail and Outlook.com are stricter
One of the primary reasons for DKIM failures with Microsoft email services, even when Gmail passes, often relates to how email content is modified or handled during transit. Unlike some other providers, Microsoft mail servers are known for their strict interpretation of email formatting and character encoding. This can lead to a "body hash did not verify" error, as discussed in various forums (e.g., ServerFault on valid DKIM failing:DKIM body hash did not verify error).
This issue typically arises if the email content, including hidden characters or slight formatting changes, is altered between signing and verification. Even minor modifications, such as converting tabs to spaces, or changes in line endings or character sets, can invalidate the DKIM signature. Some users have noted that certain content, especially non-ASCII characters or HTML entities, can be problematic.
How
Gmail processes DKIM
Tolerant of minor formatting changes, primarily checks signature validity and domain alignment.
How
Hotmail/Outlook.com processes DKIM
Strict about content integrity (body hash), sensitive to character encoding, and prone to interoperability issues with certain signing software.
Another factor could be Microsoft's more rigorous approach to DMARC alignment. Even if DKIM technically passes, if the domain in the "From" header (RFC5322.From) does not align with the domain signed by DKIM (the d= tag in the DKIM signature), the email might still face deliverability issues, especially under a strict DMARC policy. This strictness can sometimes manifest as a DKIM fail in the authentication results, even if the primary issue is alignment rather than the signature itself.
Troubleshooting common causes of failure
When DKIM fails specifically for Hotmail or Outlook.com, the first step is to meticulously examine your email content. Simplistic, plain-text emails are a good starting point for testing. Gradually add elements back in to identify if specific characters, formatting, or even the email client/sending platform are introducing changes that invalidate the signature. Ensuring your content adheres strictly to standards and avoids any unusual encoding or hidden characters can mitigate body hash issues.
Secondly, verify your DKIM DNS record. While it might pass with other providers, check for any unintended wildcards (e.g., *._domainkey) or issues with the public key. The key length and algorithm used (e.g., rsa-sha256) should also be standard and widely supported. An improperly formatted or missing public key can lead to verification failures. You can use a tool like a free email deliverability tester to inspect your email headers.
Example Hotmail Authentication-Results Headerplain
Authentication-Results: spf=pass (sender IP is 18.214.133.205)
smtp.mailfrom=kiusys.com; dkim=fail (signature did not verify)
header.d=kiusys.com;dmarc=none action=none header.from=;
Received-SPF: Pass (protection.outlook.com: domain of kiusys.com designates
18.214.133.205 as permitted sender) receiver=protection.outlook.com;
client-ip=18.214.133.205; helo=email.kiusys.com;
Received: from email.kiusys.com (18.214.133.205)
Decode authentication results
The Authentication-Results header provides crucial clues. Look for dkim=fail messages, specifically noting reasons like "signature did not verify" or "body hash did not verify." This information directly guides your troubleshooting efforts.
It is also important to consider the mail flow and any intermediate services that might modify the email. Forwarding services, for example, are notorious for altering email headers or bodies, which can break DKIM signatures. If emails are being forwarded to Hotmail or Outlook.com, the forwarding process itself could be the culprit, even if the original email passes DKIM perfectly. This is a common challenge with email forwarding.
To understand the specific reason for failure, analyze the Authentication-Results header in emails received by Hotmail or Outlook.com. These details are critical for pinpointing the exact problem. For a deeper dive into Microsoft's specific behaviors, resources like Mailgun's insights on Outlook's new rejection reasons might be helpful (see their blog post on email authentication your ID card).
Implementing solutions and best practices
Addressing DKIM failures with Microsoft requires a systematic approach. If content modification is suspected, try to simplify email templates and avoid complex formatting or characters that might be subject to alteration. Experiment with different canonicalization algorithms (relaxed versus simple) in your DKIM configuration, as relaxed canonicalization is more tolerant of whitespace and header changes.
Ensure your sending infrastructure consistently applies DKIM signatures. If you use multiple sending systems or third-party email service providers (ESPs), confirm that each one is correctly signing your emails with the appropriate DKIM key for your domain. Inconsistencies across different mail streams can lead to intermittent failures.
Algorithm
Description
Tolerance to Changes
Relaxed
Allows for minor changes in whitespace and header order.
More flexible.
Simple
Requires exact match; sensitive to any changes in headers or body.
Stricter.
If you are using an older version of OpenDKIM or similar signing software, consider updating it. While an older version might pass with Gmail, newer versions often include bug fixes and better interoperability with various email providers, including Microsoft. Sometimes, it is not your setup that is flawed, but rather the receiving server's verification process that causes the discrepancy.
Audit email flow: Identify any intermediaries between your sending server and the recipient's mailbox that might modify the email.
Test content variations: Send test emails with minimalist content to isolate if specific elements are causing the DKIM hash to break.
Monitor DMARC reports: These reports provide aggregate data on DKIM authentication results from various receivers, including Microsoft, giving you visibility into failures. A DMARC monitoring solution can help interpret these reports.
Keep DKIM keys updated: Ensure your DKIM keys are secure and have an adequate length (e.g., 2048 bits for better security and future compatibility).
Review your DMARC policy: While DKIM itself might fail, a DMARC policy with p=none can help ensure delivery while you troubleshoot the DKIM issues. However, for strong brand protection and deliverability, aiming for p=quarantine or p=reject is ideal once DKIM is consistently passing. Learn more about safely transitioning your DMARC policy.
Views from the trenches
Best practices
Regularly review email headers for Authentication-Results to identify specific DKIM failure reasons.
Test email content with simple text, then add complexity to pinpoint breaking elements.
Ensure your sending mail server's DKIM implementation uses robust canonicalization.
Common pitfalls
Ignoring DKIM failures from one provider because others show passes, impacting deliverability.
Assuming a DKIM setup working for most providers will work perfectly for all, especially Microsoft.
Not considering email forwarding as a cause for DKIM signature invalidation due to alterations.
Expert tips
For persistent issues, try generating a completely new DKIM key pair and correctly publishing it.
If using an ESP, confirm their DKIM implementation is optimized for broad compatibility.
Pay close attention to character encoding and line endings; they are frequent culprits.
Marketer view
Marketer from Email Geeks says, "There's no universal explanation or solution, but it is known that Microsoft can invalidate a DKIM signature even if it is valid."
January 19, 2022 - Email Geeks
Marketer view
Marketer from Email Geeks says, "A wildcard record in your DKIM DNS might be causing unforeseen issues, and it's worth removing it to recheck."
January 19, 2022 - Email Geeks
Ensuring consistent DKIM authentication
While DKIM failing in Hotmail (or Outlook.com) while passing in Gmail can be a frustrating experience, it often points to the nuanced differences in how email service providers implement and enforce email authentication standards. Microsoft's stricter validation, particularly concerning content modifications and interoperability, requires a more meticulous approach to your DKIM setup and email content. By understanding these distinctions, diligently troubleshooting content, verifying DNS records, and continuously monitoring your email authentication, you can significantly improve your deliverability to all recipients, including those on Microsoft platforms. Ultimately, ensuring robust DKIM authentication is a critical step in maintaining a strong sender reputation and avoiding the dreaded spam folder.
Gmail processes DKIM
Hotmail/Outlook.com processes DKIM
