The POPular POP proxy and POP servers communicate through the XPOP protocol, which is a slightly modified POP3 protocol (defined in RFC 1939 [RFC1939]). The modifications are needed for telling the POP server the mailbox to use and some more information. This extension is used only by the POPular proxy and server. If you use a different server, use the POP3 protocol.
The XPOP protocol is really simple. As soon as the connection between the proxy and the server is established the proxy sends three lines of text, each ending with a CRLF combination. The content of the lines is, in this order:
The mailbox to use. This is a directory name relative to the POP spool defined in the configuration of the POP server. It can include any number of subdirectories. It does not start with a /. No ".." is allowed in the name.
An ID string used for this connection. This ID is generated by the POP proxy and used in all logging messages on the proxy and server to be able to easily find corresponding entries.
A list of flags. Flags are denoted by ASCII chars. In the absence of the char, the flag is 'false', otherwise 'true'. Currently there is only one flag 'M' defined.
After this three lines are sent the POP server answers with '+OK' or '-ERR' according to the POP3 protocol. This message is passed through to the client. From then on everything is just like in the POP3 protocol but starting in the transaction phase, not in the authorization phase.
When configuring a backend server the protocol CXPOP can be used. This just means that the XPOP protocol should be used after checking for available mail with the MAILCHECK protocol, which is described in the next chapter.