According to RFC 1081, POP3, which is a successor to the Post Office Protocol described in RFC 918 (J.K. Reynolds, Oct-01-1984) and to the Post Office Protocol version 2 described in RFC 937 (M. Butler, J. Postel, D. Chase, J. Goldberger, J.K. Reynolds, Feb-01-1985), was designed with small systems in mind. What is a small system? A computer which is not connected continuously to the Internet or a computer which lacks enough resources (speed, RAM, bandwidth and disk space are the most common constrainers) to be a full-fledged multiuser mail server - a Message Transport System, or MTS. POP3 implements the necessary commands to allow the user of such machine to poll a larger computer at the intervals the user sees as most convenient to get the mail messages stored at the server and transfer them to his personal computer (usually, but not always, deleting them from the server). This computer needs only to be able to run a User Agent, or UA.
Nowadays, most mail is handled this way. Very few Internet users have their computer online all the time, and even fewer have the knowledge required to set up and administer a mail server. This makes POP3 one of the most important protocols existing today.
POP3 is, as SMTP, a very straightforward, line-oriented protocol. The client will open an arbitrary port and request a connection to the server's TCP port 110.
Result codes for each command are handled in the most simple way possible: If a command issued by the client succeeds, a line starting with +OK (optionally followed by a text string) is sent back. If the command fails, a line starting with -ERR (again, optionally followed by a text string) is sent back.
If a multi-line answer is to be sent (only possible with certain commands), the server will indicate the answer is over with the sequence <CR><LF>.<CR><LF> (a period on an empty line).