I finished writing a new milter for Postfix (or other MTAs) that can check the From:-header of an email and compare it with a list of given domains, if the underlying mail exchanger is a final destination. Currently it knows about two modes.

The first mode will reject a sender, if he or she sends with a foreign From:-header of another mail provider.

In the second mode, it will continue sending the mail, but it replaces the From:-header with the envelope-from (MAIL FROM) and adds a Reply-To:-header. In this mode, no information gets lost.

Why is all that important?

If mail exchangers run DMARC policies, then they do SPF and DKIM verification. If you run i.e. a web server where you have customers that have their own sides, they might provide a contact formula. Many of such formulas replace the From:-header with a email address a visitor has provided. And that will instantly break DMARC policies.

I do not want to go deeper into detail, so I encourage you to visit the projects home:

https://github.com/croessner/vrfydmn

Hope you enjoy. Feedback is very welcome