POP3 protocol

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Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

POP3 protocol

The Post Office Protocol 3 (POP3) is a standard protocol for retrieving e-mail. The POP3 protocol controls the connection between a POP3 e-mail client and a server where e-mail is stored. The POP3 service uses the POP3 protocol for retrieving e-mail from a mail server to a POP3 e-mail client.

The POP3 protocol has three process states for handling the connection between the mail server and the POP3 e-mail client: the authentication state, the transaction state, and the update state.

During the authentication state, the POP3 e-mail client that is connected to the server must be authenticated before users can retrieve their e-mail. If the user name and password that are provided by the e-mail client match those on the server, the user is authenticated and proceeds to the transaction state. If not, the user receives an error message and is not allowed to connect to retrieve e-mail.

To prevent any damage to the mail store after the client has been authenticated, the POP3 service locks the user's mailbox. New e-mail that is delivered to the mailbox after the user has been authenticated (and the mailbox has been locked) is not available to download until after the connection has been terminated. Also, only one client can connect to a mailbox at a time; additional connection requests to the mailbox are rejected.

During the transaction state, the client sends POP3 commands and the server receives and responds to them according to the POP3 protocol. Any client requests the server receives that do not comply with POP3 protocol are ignored, and an error message is sent back.

The update state closes the connection between the client and server. It is the last command the client transmits.

After the connection is closed, the mail store is updated to reflect the changes made while the user was connected to the mail server. For example, after the user successfully retrieves e-mail, the retrieved e-mail is marked for deletion and then deleted from the mail store, unless the user's e-mail client is configured to do otherwise.