Post Office Protocol version 3 (POP3)

Definition of Post Office Protocol version 3 (POP3) in The Network Encyclopedia.

What is Post Office Protocol version 3 (POP3)?

An Internet standard protocol for storing and retrieving messages from Simple Mail Transfer Protocol (SMTP) hosts.

How It Works

SMTP provides the underlying transport mechanism for sending e-mail messages over the Internet, but it does not provide any facility for storing messages and retrieving them. SMTP hosts must be continuously connected to one another, but most users do not have a dedicated connection to the Internet.

Post Office Protocol version 3 (POP3) provides mechanisms for storing messages sent to each user and received by SMTP in a receptacle called a mailbox. A POP3 server stores messages for each user until the user connects to download and read them using a POP3 client such as Microsoft Outlook 98, Microsoft Outlook Express, or Microsoft Mail and News.

To retrieve a message from a POP3 server, a POP3 client establishes a Transmission Control Protocol (TCP) session using TCP port 110, identifies itself to the server, and then issues a series of POP3 commands:

  • stat:
    Asks the server for the number of messages waiting to be retrieved

     

  • list:
    Determines the size of each message to be retrieved

     

  • retr:
    Retrieves individual messages

     

  • Quit:
    Ends the POP3 session

     

NOTE

After a POP3 client reads a message in its mailbox on a POP3 server, the message is deleted. Primarily because of this, POP3 is being supplanted by Internet Mail Access Protocol version 4 (IMAP4), which offers better support for mobile users. POP3 is supported by Microsoft Exchange Server.

TIP

To troubleshoot problems with remote POP3 servers, use Telnet to connect to port 110 and examine the results as you try issuing various POP3 commands such as the ones just described.