Post Office Protocol (POP)

POP is stands for Post Office Protocol. The POP delivers all the messages stored on the server to the user's email account. It can be configured in such a manner that the user can obtain emails from his multiple email accounts into his only email software installed on the computer such as the Microsoft Outlook or Outlook Express. This facility is also available on almost all the Internet based email website such as Hotmail, Yahoo etc.

Like SMTP, the POP is also a very simple protocol and can be easily written for a command-line based application. With the advent of a number of Windows based applications, the difficulty of writing such command-line based applications has totally vanished and the work of user has become much easier.
Once logged into the mail server, the transactions allow doing the following tasks :

STAT - Gives the number of messages in the box as well as total size of those messages in bytes
LIST - Gives the list of messages in the mailbox
RETR msg - RETR retrieves a message. It uses the message number as generated using the LIST command.
DELE msg - Deletes a message. Message would be permanently deleted with the sessions ended using the quit command. Message can be undeleted using RSET command.
TOP msg n - Returns the headers of desired message and also n lines of its body.