Based on Debian 10 "Buster" environment.

Mail

To receive and send out, MTA, local delivery, and IMAP server are required.
For the security and trustable transportation, Anti-virus, Anti-spam, and mail signing are required.
These are server level (or server itself) set up.

To handle mails more efficiently, per-user anti-spam and other filtering should be ready.

Server Level

Per-User Level