Why is DKIM failing for Hotmail but passing for Gmail and Yahoo?

Email marketer from Stack Overflow explains that Hotmail/Outlook might be using an outdated or non-standard method for checking DKIM records that leads to it failing even when the records are technically valid.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.

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.

Email marketer from Email Geeks confirms that the issue was related to duplicate headers, specifically the MIME-Version header, which was added by both ActionMailer and their API.

Email marketer from Litmus indicates that the 'd' tag in the DKIM signature should be in alignment with the domain that is being used in the 'From:' header and, that is the one being checked, if they don't match that can cause issues.

Email marketer from Neil Patel's Blog shares that DKIM failures can occur due to alterations in the email content during transit, such as changes in encoding or the addition of footers by intermediaries. Hotmail/Outlook may be more sensitive to these changes.

Email marketer from GlockApps suggests that DKIM failures can stem from inconsistent handling of email headers. Some email providers, including Hotmail, may automatically reorder headers or modify them during transmission, which can invalidate the DKIM signature if not properly handled.

Email marketer from EmailOnAcid suggests that Hotmail's servers might have aggressive spam filtering rules that inadvertently flag DKIM failures. While the signature might be technically valid, other factors such as IP reputation or content may contribute to a higher spam score, leading to DKIM checks being more strictly enforced.

Email marketer from Mailjet explains that incorrect DKIM record setup (syntax errors, incorrect key length) can lead to verification failures. Additionally, if the domain used for DKIM signing does not align with the sender domain, Hotmail might flag it as a potential issue, while Gmail and Yahoo are more forgiving.

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.

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.

Expert from Spam Resource, John Levine, explains that Hotmail is notorious for modifying email headers in transit. These modifications invalidate the DKIM signature, leading to failures even if the initial signature was valid.

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.

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.

Documentation from AuthSMTP indicates that the receiving mail server's algorithm to validate DKIM records may be the reason. If this is the case, there's no real answer why other than the mail server is the issue.

Documentation from RFC Editor details that variations in implementation of DKIM validation algorithms across different email providers can result in differing outcomes. Specifically, Hotmail may implement a more rigorous validation process than Gmail or Yahoo, causing DKIM checks to fail more often.