[SoftTalker]

My work email is Outlook, which is horribly broken and terrible to use. I have a rule configured to "re-send" all my mail to a different account where I read it with a usable MUA. Unfortunately this seems to break DMARC for external mail as now an email from e.g. [e-mail] appears to have been sent by outlook.com.

[matharmin]

I'm not familiar with Outlook's resending, but the use case is supported if the sender uses DKIM. If the email is forwarded without changing any details, it can keep the DKIM signature. That allows the forwarded email to still pass DMARC.

Now if the sender used SPF + DMARC but not DKIM, this does not work, since the sender IP can't be verified with the forwarded email. In that case, the forwarder has to change the from address to prevent the email from failing DMARC and be rejected.

In practice, senders using SPF+DMARC but not DKIM should be quite rare, you see DKIM+DMARC much more often.

[Polizeiposaune]

I have a long-standing email address that forwards to an email system that I run. The operator of the forwarder switched to using Microsoft's mail infrastructure some years ago and the quality of service of the forward has degraded dramatically ever since.

I've often seen messages resent by Microsoft's mail infrastructure with gratuitously broken DKIM signatures, generally due to changes to whitespace that are not anticipated by DKIM's message canonicalization.

I've also seen messages sent by my bank directly to the email system I administer that had broken DKIM signatures apparently due to some sort of antivirus software they had downstream of the DKIM signer.

[ryanbrunner]

Outlook.com also seems to routinely ignore DMARC (it will bounce emails with a DMARC that's report only)

[matharmin]

I never understood the point of the anti-virus adding a message to _outgoing_ emails. Basically "I swear there is no virus in this email I'm sending you, trust me bro".

[SoftTalker]

"We take security seriously."