~green offers email for all shell users, you do not need to register for an additional service
username
(no domain), password is the shell passwordManual config parameters are:
Incoming:
Outgoing:
you can also use STARTTLS with the default ports if you prefer (143, 587)
The only case in which you might need to put the domain is if you're using a mail account provided via postfixadmin, when you bring your own domain into tilde.green's mail system, the process of how to do so, requires contacting jmjl to get DNS records set up in place, and an account registered.