This article is from the LAN Mail Protocols FAQ, by John Wobus firstname.lastname@example.org with numerous contributions by others.
Also know as PCMAIL. Desktop computers can use this protocol to both
send and receive mail. The system is designed around the idea that
each user can own more than one workstation; however, the system
doesn't seem to handle the idea of a "public workstation" very well.
The desktop computers are assumed to hold state information about the
mail, a directory so to speak, and when the desktop computer is
connected to the server, this directory is updated to "reality". Note:
DMSP never gained the following of IMAP or POP and I've heard the
software is no longer available.