alt Hacker NewsMy 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. [email protected] appears to have been sent by outlook.com.
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.