IMAP (Internet Message Access Protocol), along with POP3, is one of the most widely used email transfer methods. IMAP synchronizes email account information with the mail server on a regular basis. If you log in to multiple computers to check your email, IMAP will allow you to see what messages you have viewed, replied to, forwarded, etc. POP3 does not allow you to see this information.
Note: Email storage counts against your total Disk Space limits.