Why is my DKIM failing in Microsoft but passing in Gmail and Yahoo?
Summary
What email marketers say8Marketer opinions
Email marketer from SendGrid explains the importance of SPF and DKIM alignment for passing DMARC checks. While DKIM might pass individually, failing DMARC due to alignment issues could lead Microsoft to treat the email as suspicious. They advise ensuring proper SPF and DKIM alignment to improve deliverability.
Email marketer from Email on Acid stresses the importance of testing your DKIM setup using various email clients, including Microsoft Outlook and Office 365. They advise using Email on Acid's platform to send test emails and analyze the results, helping to identify any discrepancies or configuration errors.
Email marketer from GlockApps recommends checking the IP reputation of your sending server. If your IP address is blacklisted or has a poor reputation, Microsoft may reject emails despite a valid DKIM signature. They suggest using GlockApps' IP reputation monitoring tools to identify and address any issues.
Email marketer from an Email Marketing Forum suggests that long lines in the DKIM TXT record can sometimes cause issues with DNS resolution, especially for Microsoft's servers. They recommend breaking up the DKIM record into multiple shorter lines (less than 255 characters each) to improve compatibility.
Email marketer from MXToolbox recommends checking for DNSSEC-related issues. If DNSSEC is enabled on your domain, a broken DNSSEC configuration can interfere with DKIM validation, leading to failures in Microsoft's email systems. They suggest using MXToolbox's DNSSEC tool to diagnose any problems.
Email marketer from Stack Overflow suggests that Microsoft might be more strict in DKIM validation compared to other providers. This stricter validation means even minor issues with the DKIM signature or DNS record may cause a failure. They recommend double-checking the syntax of the DKIM record and ensuring it matches the private key used for signing.
Email marketer from mail-tester.com suggests that while DKIM is important, Microsoft also analyzes the content of the email. If the email contains spam-like content or suspicious links, Microsoft might flag it even with a valid DKIM signature. They advise using mail-tester.com to analyze your email's content and identify any potential red flags.
Email marketer from Reddit suggests that Microsoft might be applying reputation filtering on top of DKIM validation. Even if the DKIM signature is technically valid, Microsoft may still flag the email as suspicious if the sending domain has a poor reputation. They advise monitoring your domain's reputation with Microsoft's SNDS program.
What the experts say3Expert opinions
Expert from Word to the Wise responds that Microsoft's DKIM implementation is known to be more sensitive to minor deviations from the RFC specifications. Specifically, they recommend ensuring that the canonicalization algorithms specified in your DKIM signature ('h=' and 'b=' tags) are correctly implemented. Microsoft's servers may be less forgiving of relaxed canonicalization than other providers.
Expert from Email Geeks shares a thread discussing why DKIM may fail at Microsoft while passing everywhere else. The URL for the thread is https://emailgeeks.slack.com/archives/C0Y3T12Q0/p1668786804486909.
Expert from Spam Resource explains that Microsoft uses feedback loops (FBLs) to track spam complaints. If a significant number of recipients flag your emails as spam, Microsoft may temporarily penalize your domain or IP address, even if your DKIM signature is technically valid. This can manifest as DKIM failures because Microsoft might be temporarily suppressing emails from your domain.
What the documentation says4Technical articles
Documentation from dkim.org lists potential issues such as incorrect key length, whitespace errors in the DKIM record, and DNS propagation delays. These factors can cause intermittent DKIM failures, particularly when Microsoft's servers check the DKIM record.
Documentation from AuthSMTP lists potential issues such as incorrect DNS record setup, DNS propagation delays, and DNS lookup failures as reasons for DKIM failure. They also explain that a failure to have the correct subdomain set up with the correct key or any typographical errors in the DKIM record can cause failure.
Documentation from RFC 6376 (the DKIM standard) explains the required format and syntax for DKIM signatures. According to the RFC, strict adherence to the format is essential for successful verification. Any deviations, such as incorrect header field names or invalid key encodings, can result in failure.
Documentation from Microsoft Docs explains that DKIM verification failures can occur if the message is altered in transit, invalidating the signature. They suggest checking the message headers for modifications and ensuring that the DKIM record is correctly published in DNS.