Why does DKIM show a permerror (bad sig) only on Yahoo Mail?

A DKIM permerror, particularly a 'bad signature' indication exclusively on Yahoo Mail, points to a fundamental failure in the email's cryptographic signature verification. This issue frequently arises not from a generalized DKIM setup problem, but from Yahoo Mail's notably stringent and sensitive validation process. Unlike other email providers, Yahoo's robust parsers are highly attuned to even minor alterations, non-standard formatting, or subtle discrepancies in an email's headers or body after the DKIM signature has been applied. Common culprits include modifications introduced by intermediate servers, such as email forwarding services or mailing lists, which invalidate the original signature without re-signing. Furthermore, Yahoo's strictness extends to precise canonicalization, meaning variations in line endings, extra whitespace, or header order can lead to rejection. Errors within the DKIM DNS TXT record itself, like incorrect public keys, are also more likely to be caught by Yahoo's detailed checks. Ultimately, this 'bad sig' often signifies that the message content or headers do not cryptographically match the original signature, an issue Yahoo is uniquely positioned to detect and enforce due to its advanced DMARC policies.

When a DKIM permerror, specifically a 'bad signature' status, appears exclusively for emails sent to Yahoo Mail, it signals a particular challenge with Yahoo's highly stringent authentication protocols. Unlike many other email providers, Yahoo's systems, underpinned by robust DMARC enforcement, are engineered to detect even the slightest alterations or formatting deviations in an email after its DKIM signature has been applied. This heightened sensitivity often means issues arise from changes introduced by intermediary services like forwarding systems or mailing lists, which inadvertently invalidate the original signature without re-signing the message. Furthermore, Yahoo's validation is notoriously strict about canonicalization, where variations in elements such as line endings, whitespace, or header order can lead to a rejected signature. Even subtle imperfections in the DKIM DNS TXT record itself, or headers not explicitly covered by the signature, are more readily identified and flagged by Yahoo, resulting in delivery failures.

A DKIM 'permerror' with a 'bad signature' status, particularly when it appears only on Yahoo Mail, indicates that the email's cryptographic signature cannot be successfully verified. Fundamentally, this means the message was either altered after being signed by your Mail Transfer Agent (MTA) or signed incorrectly. While this issue is not exclusive to Yahoo, their stringent validation processes are highly effective at detecting even subtle discrepancies. This precision often highlights problems such as modifications to email headers or body content during transit, an incorrect signing key being used, or a mismatch between the published public key and the one used for signing, all of which invalidate the email's original integrity.

A DKIM 'permerror' indicating a 'bad signature' specifically with Yahoo Mail signifies a failure in verifying the email's cryptographic integrity, meaning the received message content or headers do not match what was originally signed. While the underlying issue is a failure to maintain the email's integrity, Yahoo Mail's verification process is exceptionally rigorous, often exposing subtle discrepancies that other mail providers might overlook. These discrepancies frequently stem from alterations occurring post-signing, incorrect DKIM record configurations, or strict adherence to canonicalization rules where even minor formatting variations can invalidate the signature. This issue, though not exclusive to Yahoo, is often brought to light by their meticulous validation and robust DMARC enforcement, which are highly effective at identifying authentication failures.