The basic protocol is the following: the client issues a request (also called command in POP3 terminology) and the server responds with the result. Both commands and responses are line based, each command is sent as a complete line, a response is either a single line or - in case of mail transfer commands - multiple lines.

Commands begin with a case-insensitive keyword possibly followed by one or more arguments (such as RETR or DELE).

Responses begin with a status indicator ("+OK" or "-ERR") and a possible explanation of the status code (e.g.: "-ERR Permission denied.").

Responses to certain commands (usually mail transfer commands) also contain a data attachment, such as the mail body. See the Section 4.9.1.3, Bulk transfers for further details.