What does a mailing list need to do to reflect a message?

Method 1
========

Keep the original From header. Add a Sender header field with the list's
address. Broadcast message with MAIL FROM set to the list's address.

Issues:

* Bounces don't go to the list (they should because of Sender).
* Can't add a prefix to Subject or a footer to body. No unsubscribe link, is
  this GDPR-compliant?
* Breaks DMARC alignment for SPF (but SPF is legacy). If DKIM is also used it
  should be fine, except if the DMARC policy specifies fo=1 (requires both SPF
  and DKIM to pass).

Method 2
========

Replace From with the list, add Reply-To with the original From, strip DKIM.
Broadcast message with MAIL FROM set to the list's address.

Issues:

* Headers are messed up and don't make sense (From is not the real From).
* Replies go to the list instead of the original author.

Extra stuff
===========

* Add List-* fields
* Sign with the list's DKIM key

References
==========

* SPF RFC: https://tools.ietf.org/html/rfc7208
* DKIM RFC: https://tools.ietf.org/html/rfc6376
* https://begriffs.com/posts/2018-09-18-dmarc-mailing-list.html