There are many types of email servers which can handle a wide variety of email protocols. Some email servers are used for delivering mail, some for retrieving mail, some for relaying mail, while others are used for filtering mail, and that's not all!
Typically email servers are setup so that email can be received, sent, and accessed, and that at least requires an SMTP server and either an IMAP or POP server. The SMTP server is used for receiving and sending, while the IMAP or POP server is used for accessing the email.
Here is a listing of email protocols and the respective servers:
SMTP
- Postfix
- Exim
- Qmail