http://www.ietf.org/rfc/rfc1056.txt
Pcmail is a distributed mail system providing mail service to an arbitrary number of users, each of whom owns one or more workstations. Pcmail's motivation is to provide very flexible mail service to a wide variety of different workstations, ranging in power from small, resource-limited machines like IBM PCs to resource-rich (where "resources" are primarily processor speed and disk space) machines like Suns or Microvaxes. It attempts to provide limited service to resource-limited workstations while still providing full service to resource-rich machines. It is intended to work well with machines only infrequently connected to a network as well as machines permanently connected to a network. It is also designed to offer diskless workstations full mail service.