moderated Rewrite From field in member's own messages when "I always want copies of my own emails" is checked #suggestion
Earlier I may have opposed rewriting the From field in member's own messages when "I always want copies of my own emails" is checked, but a few factors have changed by viewpoint.
Now I think the option should control the rewriting of both the Message-ID and the From fields when a member's post is sent to their own address.
1) You've since begun using the DMARC rewriting for Gmail users when their messages go back to themselves. That was forced by Gmail's imposition of a BIG scary message on one's own messages when received from Groups.io (and presumably any other email list or forwarding).
2) "Why doesn't this feature work" (referring to that checkbox) has been an ongoing FAQ in GMF. The wording of that option has always seemed too broad to me, and almost no one understands why it only applies to Gmail users (or other Google-powered domains).
3) I've tracked down some cases where a member's email service is clearly rejecting or quarantining messages specifically because they list the member's own email address in the From. I believe the DMARC rewriting would resolve these cases.
This might allow you to eliminate some of the special cases for From rewriting for specific mail domains, when the condition can be replaced by conditioning on this checkbox. You could, as part of this implementation, automatically check the box for any account previously covered by a special case test.