When I send a multipart email from my server to outlook.com, but have it forwarded to a Gmail account (or anywhere else), DKIM fails, with the message "body hash did not verify", even though as far as I can tell the body looks identical to when I send the same message to Gmail directly, and copy-and-pasting the body part and doing a diff yields no differences as well. I have relaxed/relaxed verification, so whitespaces shouldn't be an issue.

I have no problem with any other email provider forwarding my messages, and if I send a plain text only email to outlook and have it forwarded, I have no problems.

Anyone have any idea what might be going on here? Is this a known issue with outlook.com? (I know it was when DKIM was new, but its been years now, is it possible Microsoft still hasn't fixed it?)

  • It is not fully clear what you are doing (no way to reproduce it based on your description) but it might be that either some content is added to the mail during forwarding or that the mail is recoded since parts of it where 8 bit and some server on the way did not support 8BITMIME. For a deeper explanation for the latter see Breaking DKIM by Chance (disclaimer: own research). If you have some way to reproduce it or provide the source code of the mail before and after forwarding one might have a closer look at it. – Steffen Ullrich May 30 at 3:49

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