I wish I had the courage to do it on Guix but I personally went to just hosting it in DockerMailServer on Podman running on Debian. Not sure about mail services, I looked at mailbox.org and it's probably what I would've picked if I didn't go with self hosting, but I can't comment on the experience as I haven't tried it myself.
@boo_ @shegeley My emails have been at mailbox.org since 2017 or so, and it has been a very smooth experience. No surprises. Reasonable pricing too. I can highly recommend!
With a custom domain, you need some configuration to make sure your emails are not marked as spam by the big tech email providers; mailbox.org has good documentation about it.
@shegeley self-host? Then @mwl has got you covered with his Run Your Own Mail Server book https://lwn.net/Articles/998153/
https://www.amazon.com/Run-Your-Mail-Server-Mastery/dp/1642350796
There is https://codeberg.org/whitequark/mail.whitequark.org in nix, maybe you can covert to guix.
For provider