[Mailman-Developers] Fixing DMARC problems with .invalid munge

[John Levine]

I realize I'm a bit late to this party, but this is a technique that I
don't think has been addressed here.  On my lists I've fixed the DMARC
bounces by rewriting From: lines of DMARC'ed domains like this on the
way out:

  From: Marissa <mmeyer at yahoo.com>

to

  From: Marissa <mmeyer at yahoo.com.invalid>

Before you tell me I'm nuts, hear me out.  I've actually implemented
this, and it works.

Advantages:

* Really fixes DMARC problems

* Everyone can still post to lists

* Doesn't change the way users in non-DMARC'ed domains work

* Leaves the author's address in the headers, so you can reply and use
  MUA sorting rules, give or take usenet-style unmunging.  (This is a
  reason we dislike putting the list address in the From: line.)

* Code is simple

* Gives people a hint of where the actual problem lies

R's,
John

PS: I use .invalid because RFC 6761 promises it will never be
allocated to anyone.