If you have configured Sendmail as a Central Mail Hub Server, you must install IMAP/POP software or you'll not be able to take advantage of your Linux Mail server since Sendmail is just software that sends mail from one machine to another, and nothing else. A mail server is a server that is running one or more of the following:

An IMAP server

A POP3 server

A POP2 server

or an SMTP server.

An example of SMTP server is Sendmail that must be already installed on your Linux server as a Central Mail Hub before you continue reading this part of the book. For now, we are going to cover installing IMAP4, POP3, and POP2, which all come in a single package.