SMTP, POP3, and IMAP4

The following is a list of the different protocols that the Internet uses to transfer and receive mail:

SMTP The Simple Mail Transfer Protocol (SMTP) is used to send e-mail. SMTP travels over TCP port 25, and is used by clients to send messages.

POP3 Post Office Protocol version 3 (POP3) is one of the two protocols that receive e-mail from SMTP servers. POP3 uses TCP port 110. Most e-mail clients use this protocol, although some use IMAP4.

IMAP4 Internet Message Access Protocol version 4 (IMAP4) is an alternative to POP3. Like POP3, IMAP4 retrieves e-mail from an e-mail server. IMAP4 uses TCP port 143 and supports some features that are not supported in POP3. For example, IMAP4 enables you to search through messages on the mail server to find specific keywords, and select the messages you want to download onto your machine. IMAP4 also supports the concept of folders that you can place on the IMAP4 server to organize your e-mail. Some POP3 e-mail clients have folders, but that's not a part of POP3, just a nice feature added to the client.