Mail servers come in different forms.
- Mail Transfer Agents (MTA) typically speak SMTP and perhaps LMTP.
- Mail Delivery Agents (MDA) speak IMAP, POP3 usually also supporting SSL/TLS (IMAPS, POP3S).
These mail servers can do some or all of the above.
These are add-ons to mail servers that enhance the functionality.
- SpamAssassin - analyze & score message content using bayesian filters
- Amavisd-new - meta-wrapper for spamassassin and a large number of anti-virus programs
- Sieve - a plugin for Dovecot to filter messages just before delivery
- Postgrey - a grey-listing agent for Postfix
- Procmail - a powerful filter/classifier utility