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.
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.
There exist extensions to the protocol that are supposed to fix this problem. popa3d does not support them yet, partly because this isn't going to fully fix the problem. In fact, APOP and the weaker defined SASL mechanisms such as CRAM-MD5 may potentially be even less secure than transmission of plaintext passwords because of the requirement that plaintext equivalents be stored on the server.
This manual page is based heavily on the one Camiel Dobbelaar wrote for OpenBSD.