http://www.ietf.org/rfc/rfc2449.txt
This extension to the POP3 protocol is to be used by a server to express policy descisions taken by the server administrator. It is not an endorsement of implementations of further POP3 extensions generally. It is the general view that the POP3 protocol should stay simple, and for the simple purpose of downloading email from a mail server. If more complicated operations are needed, the IMAP protocol [RFC 2060] should be used. The first paragraph of section 7 should be read very carefully.