Alternatives:
FreeLists.org
Gaggle.email (not happy that they genericize the word Listserv)
Top contenders:
Mailfence - Belgian, e3.50/mo
Mailbox.org - German, e3/mo; e1/mo plan only allows custom domain for "team accounts" (more than one account? does the first one have to do the e3 plan?)
Posteo - German, no personal domains
Runbox - Norwegian, inexpensive
Soverin: Dutch, € 3.25/mo
Protonmail or Kolab - Swiss, a bit steep; Protonmail e4.00/mo paid annually
Infomaniak: e1.50/mo (..?)
Pobox: US-based, seems like only the expensive account is for custom domains
Msgsafe.io: defunct
Countermail: Swedish, ~$4/mo
Disroot: Dutch, free but requires donation to use custom domain
Spaceship may not be super-private, but it's economical and US-based
Porkbun: Oregon, $24/year but only 20 max addresses
Getting the syntax right is hard! And it's hiding under /usr/local/cyrus/sbin/
https://www.linuxquestions.org/questions/linux-software-2/cyrus-ipurge-not-working-317640-print/
[adding a verified avatar logo using open protocols, for mail received in Gmail]
There's gotta be a way to make Majordomo cooperate, where it doesn't modify the message after being signed.
based around dovecot, but has the basic structure
This might be useful to let some less-certain spam detection to put things into the Spam folder, but hide the more obvious stuff completely.
This is the one I installed in the 90's on my home machine, and used in conjunction with fetchmail to retrieve messages over POP. Always the rebel, I had to try something besides mail and pine, and elm seemed too similar to both.
Free Email certificates from COMODO. Our free email certificates with digital signature ensure confidentiality and privacy. Get free (S/MIME) Certificates.
[LetsEncrypt and ZeroSSL certificates are only for domain validation (web/server encryption).]