Understanding POP and IMAP:
sendmail provides support for SMTP—the standard mail transport protocol for TCP/IP networks. When sendmail is configured to run as a daemon, it listens to the network and collects incoming mail. That mail can be forwarded, as directed by the /etc/aliases file, or stored locally in the user's mailbox in /var/mail. If the mail is stored locally on the server, the user can log in to the server and read the mail there. However, most users prefer to read their mail on their personal computers. Yet they don't want to run sendmail on those systems to collect mail in real time because of the complexity of sendmail and because they want to be able to take those systems offline at any time. DNS, sendmail, and the mailbox protocols combine to give the users what they want. MX records in DNS route mail to the mail server. sendmail, running as a daemon, collects and stores the mail on the server. The mailbox protocols move the mail from the server to the user's system when the user is ready to read the mail. POP and IMAP are the two protocols most commonly used to move mail from the server to the user's computer.