SMTP, an Internet protocol documented in RFC 2821 and using TCP port 25, is used by Mail Transfer Agents (like sendmail(8)) to send mail from one host to another. Most Internet users using an ISP's mail servers will use SMTP to send mail through their ISP's server. To receive e-mail, most Internet users will instead use one of the below protocols (POP or occasionally IMAP) in order to retrieve their mail from their ISP's mail server.
POP allows Internet users to download their mail messages from a central mail server to their own local machine, using a Mail User Agent (like pine(1) or Eudora). Once the messages are retrieved, they are typically (but not always) deleted from the remote server.
IMAP (like POP) allows Internet users to retrieve mail messages from a central mail server (like that provided by RocketMail or an Internet Service Provider like EarthLink or NetCom). However, unlike POP, IMAP allows multiple Mail User Agents to access the remote mailbox simultaneously, so that messages can be managed on the central server instead of on the client. This makes synchronization of mail among multiple client devices (like a desktop, a laptop, and a PalmPilot) much easier. However, as of this writing (June 2003), IMAP has not been implemented by that many ISPs, especially smaller ones.