Based on Debian 11 "Bullseye" environment.

Mail

To receive and send out, MTA, local delivery, and IMAP server are required.
For security and trustable transportation, Anti-virus, Anti-spam, and mail signing are required.

Setting up and keep running an email server is getting difficult to prevent the flow of spam emails on the internet.
It is required to understand and integrate with other systems, not just the email.