MicrosoftMicrosoftWe have found a solution!
After analysing the affected emails, which were blocked by the Microsoft servers with "DKIM=Fail", we noticed that these emails all have a Content-Transfer-Encoding of 8bit. These mails are not sent from Exchange servers, but automatically via another third-party software for customer communication (with MailKit framework).
We then opened a support case at Microsoft and received the following response/solution:
Issue Resolution: Microsoft Outlook servers may have trouble handling 8bit encoding when it comes to DKIM validation. One workaround is to change the Content-Transfer-Encoding header to quoted-printable, which seems to resolve the issue. Additionally, there have been reports that non-ASCII characters in the email body can also cause DKIM failures.
Subsequently, we have changed the email template for mail-creation in the third-party software to Base64 Content-Transfer-Encoding and now have no more problems with the Microsoft servers.
Perhaps this will help some of those affected.
Ours are all "Content-Transfer-Encoding: quoted-printable" and still failing DKIM.
Microsoft is experiencing two types of authentication validation issues: the first, consistently reproducible, is DKIM validation of 8-bit bodies or headers; the second, random, is DNS timeouts. It appears you are only affected by the second issue.
