Summary
DKIM failing in Hotmail while passing in Gmail is a multifaceted issue stemming from variations in how email providers implement and interpret DKIM, as well as several potential technical and configuration problems. Hotmail often exhibits stricter DKIM validation, is more sensitive to content modifications during transit, and might be influenced by sender reputation and SPF/DKIM alignment more than Gmail. Interoperability issues between different DKIM implementations, particularly with OpenDKIM, along with DNS propagation inconsistencies, improper DNS configuration, incorrect selector usage, key size discrepancies, and differences in how spam filters interact with DKIM results, all contribute to these divergent outcomes. Fundamentally, each receiver sets its own DKIM validation policy, making consistent results across providers challenging.
Key findings
- Validation Strictness: Hotmail/Outlook employs stricter DKIM validation policies compared to Gmail.
- Content Modification: Hotmail may alter email content, leading to DKIM signature invalidation.
- Interoperability Issues: Problems exist between Microsoft's systems and certain DKIM implementations like OpenDKIM.
- SPF/DKIM Alignment: Hotmail might prioritize stringent SPF/DKIM alignment.
- DNS Propagation: Regional DNS propagation delays can affect DKIM record visibility for Hotmail.
- Spam Filter Interaction: Hotmail integrates DKIM validation more closely with its spam filtering mechanisms.
- Reputation Impact: Sender reputation affects DKIM validation in Hotmail more significantly.
- Config & Standards: Varying interpretations of DKIM standard and configurations affects the validation.
- DKIM Control: Each provider sets its DKIM policy, impacting results.
Key considerations
- Test Configurations: Use email testing tools to evaluate DKIM outcomes on Hotmail and Gmail separately.
- OpenDKIM Compatibility: Verify that OpenDKIM and Microsoft systems work well with each other.
- DNS Verification: Check and validate that the DKIM record is accessible externally and DNS propagation has completed for Hotmail's DNS servers.
- Content Integrity Maintenance: Minimize or eliminate email content changes during transit.
- Monitor Reputation: Sustained monitoring of the sender's reputation is required.
- Record Check: Confirm DNS records are appropriately set, removing wildcard records.
- Key size: Key size should be compatible for both providers
What email marketers say
10 marketer opinions
DKIM failures in Hotmail while passing in Gmail can stem from several factors. Hotmail often employs stricter validation rules, is more sensitive to content modification, and might be influenced by sender reputation and SPF/DKIM alignment. Differences in DKIM implementation, key size requirements, DNS propagation, and the presence of specific characters or encoding issues in email content also play a role. The interaction with spam filters and handling of forwarded emails can further contribute to these discrepancies.
Key opinions
- Validation Differences: Gmail may be more lenient in DKIM validation compared to Hotmail/Outlook.
- Content Modification: Hotmail might modify email content in transit, invalidating DKIM signatures.
- SPF/DKIM Alignment: Hotmail may be stricter on SPF and DKIM alignment than Gmail.
- DNS Propagation: Regional DNS issues could lead to inconsistent DKIM record visibility.
- Spam Filter Interaction: Hotmail integrates DKIM checks more closely with its spam filters.
- Sender Reputation: Hotmail might weigh sender reputation more heavily when validating DKIM.
Key considerations
- DKIM Record Review: Ensure the DKIM record is properly configured and meets recommended standards.
- Content Testing: Test email content for specific characters or encoding issues that may trigger DKIM failures.
- DNS Configuration: Verify DNS propagation and ensure Hotmail's DNS servers can access the DKIM record.
- Email Forwarding: Consider the impact of email forwarding on DKIM validation.
- Provider-Specific Testing: Use email testing tools specific to Hotmail and Gmail to diagnose issues.
- Key size: Make sure your key size is compatible with both providers
Marketer view
Email marketer from Email Marketing Tips Forum suggests the possibility of regional DNS propagation issues. Some DNS servers used by Hotmail might not have fully propagated the DKIM record changes, while Gmail's DNS servers might be up-to-date. This can lead to DKIM passing in Gmail but failing in Hotmail.
24 Mar 2022 - Email Marketing Tips Forum
Marketer view
Email marketer from Email Deliverability Blog responds that different email providers use varied DKIM validation algorithms. Use email testing tools specific to each provider to diagnose issues. Hotmail might require stricter adherence to the DKIM standard than Gmail.
29 Jun 2023 - Email Deliverability Blog
What the experts say
4 expert opinions
DKIM failures in Hotmail, while passing in Gmail, can be attributed to several expert-identified factors. These include Microsoft's potential issues with OpenDKIM interoperability, DNS visibility problems, content modification during transit, and a heightened sensitivity to sender reputation. Ultimately, each provider dictates its own DKIM validation policies, which directly influences the acceptance or rejection of DKIM signatures.
Key opinions
- OpenDKIM Issue: Microsoft may have interoperability issues with OpenDKIM.
- DNS Problems: DNS issues can cause Microsoft to be unable to see the DKIM record.
- Content Changes: Hotmail may modify content, invalidating the DKIM signature.
- Reputation Sensitive: Hotmail may be more sensitive to sender reputation.
- Policy Differences: DKIM validation policies are determined by the receiver.
Key considerations
- Test with OpenDKIM: Ensure OpenDKIM interoperability is configured correctly.
- Check DNS Records: Verify DNS records are visible and accurate for Microsoft.
- Monitor Content: Monitor email content to prevent modification during transit.
- Monitor Reputation: Maintain and monitor sender reputation.
- Understand Receiver Policies: Understand that the receiving provider has final say over DKIM validity.
Expert view
Expert from Email Geeks suggests checking for DNS issues where Microsoft can't see the DKIM record, or content issues where Microsoft modifies the email content before checking the signature.
26 Mar 2024 - Email Geeks
Expert view
Expert from Word to the Wise responds that Hotmail/Outlook might be more sensitive to sender reputation. If your IP address or domain has a poor reputation, Hotmail may be more likely to fail DKIM checks as an additional layer of scrutiny, while Gmail might weigh reputation less heavily.
26 Oct 2023 - Word to the Wise
What the documentation says
5 technical articles
DKIM failures in Hotmail while passing in Gmail, according to documentation, often stem from content alteration in transit, invalid DKIM signatures, improper DNS configuration or propagation issues, interoperability problems between DKIM implementations, inconsistent DKIM implementation, or variations in interpreting the DKIM standard, and incorrect selector usage or DNS misconfigurations.
Key findings
- Content Alteration: Message content alteration during transit can cause DKIM failure.
- Invalid Signature: An invalid DKIM signature leads to DKIM failure.
- DNS Configuration: Improper DNS configuration is a common cause of DKIM failure.
- Interoperability: Interoperability issues between DKIM implementations can cause failures.
- Inconsistent Implementation: Inconsistent DKIM implementation can result in validation differences.
- Standard Interpretation: Variations in interpreting the DKIM standard can lead to inconsistencies.
- Selector Usage: Incorrect selector usage contributes to DKIM failures.
Key considerations
- Content Integrity: Ensure message content is not altered during transit.
- Signature Validation: Validate and verify the DKIM signature's integrity.
- DNS Setup: Properly configure DNS records for DKIM.
- DKIM Compatibility: Ensure interoperability between DKIM implementations.
- Standard Adherence: Adhere to the DKIM standard for consistent implementation.
- Testing Across Providers: Test configurations across different providers to identify sender or receiver end issues.
Technical article
Documentation from RFC 6376, the DKIM standard, explains that variations in interpreting the standard by different email providers can lead to inconsistencies. Issues like canonicalization, header field handling, and signature processing can differ, causing DKIM failures in some environments but not others.
26 May 2023 - RFC Editor
Technical article
Documentation from Port25 explains that DKIM failures can be due to incorrect selector usage or DNS misconfigurations. It also highlights that testing with different providers using the same configuration helps identify if the issue is on the sender's end or the receiver's end.
26 Oct 2022 - Port25
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