Email selfhosting software you can consider:
Boost is very appreciated.
iRedMail
https://www.iredmail.org
Mail in a Box
https://mailinabox.email
Mailcow
https://mailcow.email
Modoboa
https://modoboa.org
Poste.io
https://poste.io
Wildduck
https://wildduck.email
Mailslurper
https://www.mailslurper.com
Docker MailServer
https://github.com/docker-mailserver/docker-mailserver
Lavabit Magma
https://github.com/lavabit/magma
Yunohost GNU/Linux
https://yunohost.org
All of them are #FOSS.
@ademalsasa
Simple NixOS MailServer
https://gitlab.com/simple-nixos-mailserver/nixos-mailserver