SNM supports DMARC reporting, but it's disabled by default. For email
greybeards, that's fine, but I think it would be useful to teach email newbies (as I was a few
months ago) that this is something you should seriously consider
enabling.
I opted to put this in a new "Advanced Configurations" section that
points experienced mailserver admins to our howto guides, and newbies to
a couple of important things.
refs: https://github.com/NixOS/infra/pull/635
Add a certificate scheme for using an existing ACME certificate without
setting up Nginx.
Also use names instead of magic numbers for certificate schemes.
