Summary
DKIM failures in Hotmail while passing in Gmail and Yahoo are multifaceted. Root causes range from email structure flaws (like duplicate MIME headers) and DNS inconsistencies to Hotmail's unique email processing and aggressive filtering. Hotmail is known to modify email headers, implement stricter DKIM validation, and has a filtering system heavily influenced by content, IP reputation, and sender history. Validation discrepancies and the algorithm used by Hotmail's servers may also contribute.
Key findings
- Email Structure Issues: Invalid email structure (e.g., duplicate headers) can cause DKIM to fail, especially with Hotmail's validation process.
- Header Modification: Hotmail modifies email headers in transit, invalidating DKIM signatures, and is a known issue.
- Stricter DKIM Validation: Hotmail/Outlook employs stricter DKIM validation policies compared to Gmail and Yahoo.
- Aggressive Spam Filtering: Hotmail's aggressive spam filtering can flag emails even with valid DKIM signatures, based on factors like content and IP reputation.
- DKIM Record Inconsistencies: Inconsistencies in DKIM DNS records, such as incorrect syntax or whitespace, can lead to failures more often in Hotmail.
- Server-Side Algorithms: The DKIM validation algorithm on Hotmail's servers can be the root cause, particularly if it's outdated or non-standard.
- DKIM Key Length Sensitivity: Hotmail may be sensitive to the DKIM key length; shorter keys might fail despite passing on other platforms.
- DKIM Alignment Issues: If the DKIM 'd' tag does not align with the 'From:' header domain, deliverability problems will occur.
- DNS Issues: Temporary DNS issues may cause intermittant failures.
Key considerations
- Enforce Valid Email Structure: Ensure emails adhere to strict email structure standards to prevent Hotmail from 'fixing' and breaking DKIM.
- Monitor and Manage Headers: While difficult to control, be aware of and monitor header modifications by Hotmail.
- Prioritize DKIM Alignment: Make sure that DKIM 'd' tag aligns with the 'From:' header domain.
- Maintain Good Sender Reputation: Focus on improving sender reputation and content quality to overcome Hotmail's aggressive spam filtering.
- Validate DKIM Records: Thoroughly validate DKIM DNS records for any inaccuracies.
- Choose Appropriate Key Lengths: Consider using longer DKIM key lengths to satisfy Hotmail's security requirements.
- Test DKIM Implementation: Test DKIM implementation specifically with Hotmail/Outlook to identify and resolve compatibility issues.
- Address DNS Issues: Ensure proper DNS health to avoid intermittent issues.
What email marketers say
8 marketer opinions
DKIM failures on Hotmail while passing on Gmail and Yahoo can be attributed to several factors. These include Hotmail's sensitivity to email content alterations during transit (such as changes in encoding or added footers), incorrect DKIM record setups, inconsistent handling of email headers, and shorter DKIM key lengths. Furthermore, Hotmail may employ outdated DKIM checking methods and aggressive spam filtering, inadvertently flagging valid signatures. Ensuring DKIM signature alignment with the 'From:' header domain is also critical.
Key opinions
- Header Sensitivity: Hotmail is highly sensitive to changes in email headers during transit, which can invalidate DKIM signatures.
- Record Errors: Incorrect DKIM record setups, like syntax errors or incorrect key lengths, are a common cause of failure.
- Key Length: Hotmail may enforce stricter DKIM key length policies; shorter keys might fail on Hotmail but pass on other providers.
- Outdated Methods: Hotmail may use outdated or non-standard methods for DKIM checking, leading to false negatives.
- Aggressive Filtering: Hotmail's aggressive spam filters might flag emails with valid DKIM signatures due to other factors like IP reputation.
- DKIM Alignment: The DKIM signature's 'd' tag must align with the domain in the 'From:' header.
- Duplicate Headers: Duplicate headers, such as MIME-Version, can cause DKIM validation failures.
Key considerations
- Test Key Lengths: Test different DKIM key lengths to ensure compatibility with Hotmail's security policies.
- Monitor Header Changes: Monitor and prevent alterations to email headers during transit that could invalidate DKIM signatures.
- Review Record Setup: Thoroughly review DKIM record syntax, key length, and alignment to prevent errors.
- IP Reputation: Maintain a good IP reputation to avoid triggering Hotmail's aggressive spam filters.
- Check DKIM Alignment: Ensure the DKIM 'd' tag matches the domain in the 'From:' header for proper alignment.
- Eliminate Duplicate Headers: Ensure no duplicate headers are present in the email, as they can cause DKIM validation to fail.
- Utilize Testing Tools: Use email deliverability testing tools to identify potential DKIM issues before sending campaigns.
Marketer view
Email marketer from Stack Overflow explains that Hotmail/Outlook might be using an outdated or non-standard method for checking DKIM records that leads to it failing even when the records are technically valid. This can be due to Microsoft not always adhering to the latest DKIM standards.
11 Mar 2023 - Stack Overflow
Marketer view
Email marketer from Reddit mentions that he has observed that Hotmail can be particularly sensitive to the length of the DKIM key. Shorter keys (e.g., 512-bit) may pass on Gmail and Yahoo but fail on Hotmail due to stricter security policies.
23 Apr 2023 - Reddit
What the experts say
3 expert opinions
DKIM failures on Hotmail, while passing on Gmail and Yahoo, can be attributed to Hotmail's unique handling of emails. This includes fixing 'invalid' email structures (which then breaks the DKIM), modifying headers in transit which invalidates signatures, and a more sensitive filtering system influenced by content, IP reputation, and sender history.
Key opinions
- Email Structure Fixes: Hotmail's attempts to 'fix' slightly invalid email structures can inadvertently break DKIM signatures.
- Header Modification: Hotmail is known to modify email headers during transit, invalidating DKIM signatures.
- Sensitive Filtering: Hotmail's filtering system is more sensitive to factors beyond DKIM, including content, IP reputation, and sender history.
Key considerations
- Email Structure Validation: Ensure email structures are strictly valid to avoid Hotmail's 'fixes' that can break DKIM.
- Header Monitoring: Monitor email headers to detect modifications during transit by Hotmail, though you may not be able to prevent them.
- Holistic Approach to Deliverability: Focus on improving overall deliverability factors, including content, IP reputation, and sender history, as DKIM failure may be a symptom of broader filtering issues.
Expert view
Expert from Email Geeks suggests that the email structure might be slightly invalid, causing Hotmail to fix it and break DKIM. He identifies two MIME-Version headers as the cause, which is invalid.
30 May 2024 - Email Geeks
Expert view
Expert from Word to the Wise, Laura Atkins, shares that Hotmail/Outlook's filtering system is known to be more sensitive to various factors beyond just DKIM. A combination of content, IP reputation, and sender history influences deliverability, and DKIM failures may be a symptom of a broader filtering issue rather than the root cause itself.
8 May 2023 - Word to the Wise
What the documentation says
4 technical articles
DKIM failing on Hotmail while passing on Gmail and Yahoo can be attributed to multiple factors related to DNS, validation policies, and server algorithms. Microsoft documentation highlights that temporary DNS issues and stricter DKIM policies on Outlook can lead to failures for minor discrepancies. RFC Editor details variations in DKIM validation implementation among providers, with Hotmail potentially using more rigorous processes. DKIM.org points out that subtle whitespace or differences in the DNS record can cause errors, with Hotmail enforcing these more strictly. AuthSMTP suggests the receiving mail server's algorithm itself could be the issue.
Key findings
- Temporary DNS Issues: Temporary DNS problems can intermittently cause DKIM verification to fail.
- Stricter DKIM Policies: Outlook/Hotmail might have stricter DKIM validation policies than Gmail and Yahoo.
- Variation in Implementation: Different email providers can implement DKIM validation algorithms differently, resulting in differing outcomes.
- Whitespace and Subtle Differences: Whitespace or subtle differences in the DKIM DNS record can lead to validation errors, especially with Hotmail.
- Receiving Server Algorithm: The receiving mail server's algorithm for validating DKIM records might be the underlying cause.
Key considerations
- Monitor DNS Health: Regularly monitor DNS health to minimize potential intermittent DKIM failures.
- Comply with Stricter Policies: Ensure emails comply with potentially stricter DKIM policies enforced by Hotmail/Outlook.
- Account for Implementation Variation: Be aware that DKIM validation can vary across providers, so testing with Hotmail is crucial.
- Ensure Record Accuracy: Carefully check the DKIM DNS record for any whitespace or subtle differences that could cause errors.
- Accept Server-Side Issues: Recognize that issues with the receiving mail server's DKIM validation algorithm might be beyond control.
Technical article
Documentation from DKIM.org explains that whitespace or other subtle differences in the DKIM DNS record compared to how the email is signed can lead to validation errors. These errors may be more strictly enforced by Hotmail's servers.
28 Dec 2021 - DKIM.org
Technical article
Documentation from Microsoft Docs explains that temporary DNS issues can cause DKIM verification to fail intermittently. Outlook might also have stricter DKIM policies than Gmail or Yahoo, leading to failures when minor discrepancies exist.
7 Apr 2022 - Microsoft Docs
Can email signatures, especially via Exclaimer, cause SPF or DKIM failures and impact email delivery?
Do SPF and DKIM records need to be aligned for all email service providers?
How do I fix DKIM alignment errors and configure DKIM signing for a custom domain in Microsoft 365 and is include:spf.mtasv.net required for mailchimp?
How do I fix DKIM failing body hash verification?
How do SPF, DKIM, and DMARC email authentication standards work?
How to deal with a failing DMARC email authentication protocol?
