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.

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

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".