1. Advise on linux distribution that mail would sit on.
2. Advise on some kind of control panel so end-user could add mail accounts. (we are using zpanel, if you have better solution we can discuss).
3. Features that mail server must have:
- AntiSpam (amavis, policyd, greylist, dnsbl... something better?)
- Antivirus (clamav.. other?)
- Fail2Ban for protection.
- End-users are able to setup out-of-office reply by themselves in webmail.
- POP3 access will be disabled, only IMAP should be used. (All mails are kept on server, and should be visible on all devices user has)
- Test mail server will use subdomain (like test.domain.com)
- You should make sure that outgoing mail has good score in other antispam systems. (google, outlook.com, others..)
- Everything double-checked.