popa3d8

Usage

Overview

popa3d is a Post Office Protocol version 3 (POP3) server.

A POP3 server operates on local mailboxes on behalf of its remote users. Users can connect at any time to check their mailbox and fetch the mail that has accumulated. The advantage of this "pull" approach is that any user with a simple POP3-capable mail reader program can receive mail, eschewing the need for a full-fledged Mail Transfer Agent (MTA) and a permanent network connection.

Note that POP3 can only be used to retrieve mail, not to send it. To send mail, the SMTP protocol is commonly used.

For access to a mailbox through POP3, the username must be in the password database. Additionally, popa3d does not permit null passwords and will refuse to serve mail for root (UID 0) users.

Options

-D

Standalone server mode. In this mode, popa3d will become a daemon, accepting connections on the pop3 port (110/tcp) and forking child processes to handle them. This has lower overhead than starting popa3d from an inetd equivalent (which popa3d assumes by default) and is thus useful on busy servers to reduce load. In this mode popa3d also does quite a few checks to significantly reduce the impact of connection flood attacks.

-V

Print version information and exit.

Commands

A normal POP3 session progresses through three states: AUTHORIZATION, TRANSACTION, and UPDATE.

After the TCP connection opens, the client must authenticate itself to the server during the AUTHORIZATION state. The following commands are supported in the AUTHORIZATION state (all command names are case-insensitive).

When authorization is successful, the server enters the TRANSACTION state. The client can now list and retrieve messages or mark messages for deletion. The following commands are supported in the TRANSACTION state.

When the client issues the QUIT command in the TRANSACTION state, the server enters the UPDATE state. All messages that were marked for deletion are now removed. The server then closes the connection.