The Internet Message Support Protocol (IMSP) is designed to support the
provision of mail in a medium to large scale operation. It is intended to
be used as a companion to the IMAP4 protocol [IMAP4], providing services
which are either outside the scope of mail access or which pertain to
environments which must run more than one IMAP4 server in the same mail
domain. The services that IMSP provides are extended mailbox management,
configuration options, and address books.

WWW: http://asg.web.cmu.edu/cyrus/