IMAP (Internet Message Access Protocol) is a standard protocol for accessing e-mail from your local server. IMAP (the latest version is IMAP Version 4) is a client/server protocol in which e-mail is received and held for you by your Internet server. You (or your e-mail client) can view just the heading and the sender of the letter and then decide whether to download the mail. You can also create and manipulate multiple folders or mailboxes on the server, delete messages, or search for certain parts or an entire note. IMAP requires continual access to the server during the time that you are working with your mail.
A less sophisticated protocol is Post Office Protocol 3 (POP3). With POP3, your mail is saved for you in a single mailbox on the server. When you read your mail, all of it is immediately downloaded to your computer and, except when previously arranged, no longer maintained on the server.
IMAP can be thought of as a remote file server. POP3 can be thought of as a "store-and-forward" service.
POP3 and IMAP deal with the receiving of e-mail from your local server and are not to be confused with Simple Mail Transfer Protocol (SMTP), a protocol used for exchanging e-mail between points on the Internet. Typically, SMTP is used for sending only and POP3 or IMAP are used to read e-mail.