Read up on Postfix (SMTP) and Dovecot (POP3 and IMAP). (Sendmail is
available as an alternative to Postfix.)
Additional services you'll want include MIMEDefang (general filter
management), SpamAssassin (anti-spam), and ClamAV (anti-virus scanning).
Other useful services include OpenDKIM (validates that mail really came
from the domain it claims to have come from), Roundcube (web-based email
client), and Certbot (acquire a LetsEncrypt cert for your mail servers).
You might want OpenLDAP for authentication, identity management, and
address books. Fail2ban can help keep the script kiddies out of your
services.
Running a mail server requires knowledge of many different services and
awareness of many security issues. You have your work cut out for you.
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