For my mail server I use dovecot. By default this only binds its sockets in IPv4. It is however trivial to enable binding of IPv6 as well. Simply change the "listen = *" entry in its configuration (/etc/dovecot/dovecot.conf) as below:
listen="*, [::]"With that done and the service restarted my email if now available over IPv6 too. This should become the default most likely.