Server mail setup?

[Christopher Lemmer Webber]

Self-hosting e-mail is a terrible pain.  Nonethless I am (hopefully!!!)
moving all my servers over to Guix, and that also means moving over my
email setup.

Previously I've used a very ad-hoc setup with Postfix and Dovecot
listening on localhost-only (since configuring these things safely is so
hard).  It's been a long time since I set them up though... we don't
currently have Postfix in Guix, though Gábor Boskovits has a wip-postfix
branch so maybe I should check that out.  I'd also be fine running Exim
but I don't even know where to get started on configuring it.

Has anyone published Guix-as-a-mailserver configuration somewhere
they're willing to share and for me to peek at?

Thanks!
 - Chris