Probably user@example.com, but that’s configurable. But, you don’t have to guess. The username is listed on the Edit Users page (it says “IMAP/POP3/FTP Login”).
If you’ll be using TLS, it needs to be a name you have a certificate for. I use the “main” name for the server for all mail. Until SNI is supported by all services, it’s just simpler (it’s probably simpler and more reliable, anyway). i.e. I have a cert for virtualmin.com that I’ve use the “Copy to Postfix” and “Copy to Dovecot” buttons for. So, that’s my “main” domain, and the one I use to connect to my server. But, if I had a mail.virtualmin.com cert, I could use that one instead.
No different for SMTP, though it could be. But, again, one “main” domain for mail is the most sensible thing. Don’t make it complicated.
143 and 587 work fine. So do all the other common mail ports.
Authentication should be PLAIN. It’ll be encrypted over the wire, assuming you’re connecting via an encrypted protocol. None of the password hash types supported by email matches up with those supported by Linux system users by default, so we don’t try to use hashed passwords for mail (it’d require a separate password database…more complexity, more ways to fail in an already alarmingly complex system).