The Post Office Protocol allows computers to retrieve messages from mail server mailboxes. A computer running a mailer (mail client) application connects to the mail server computer and provides account (user) name and the password. If access to the specified user account is granted, the mail application sends protocol commands to the mail server. These protocol commands tell the server to list all messages in the mailbox, to retrieve certain messages, or to delete them. When a server receives a request to retrieve a message, it sends the entire message to the mail client. The mail client may choose to retrieve only the first part of the message.
The POP3 protocol does not provide direct support multi-mailbox Accounts. If a client application specifies a multi-mailbox Account, the INBOX Mailbox is opened.
When the client application sends a request to delete a message from the Mailbox, the message is not deleted immediately, but it is marked by the server. Only when the client application ends the session properly and closes the connection, the marked messages are then removed.
The POP module supports the XTND XMIT extension of the POP protocol. This extension allows users to submit messages via the POP protocol instead of the SMTP protocol.