Traditional computer mail systems involve a co-resident User Agent (UA) and Mail Transfer System (MTS) on a time-shared host computer which may be connected to other hosts ina network, with new mail posted or delivered directly through co-resident mail-slot programs. To introduce personal micro-computers (PCs) into this environment requires modification of the traditional mail system architecture. To this end, the MZnet project uses a split-slot model, placing UA programs on the PCs while leaving MTA programs on a mail relay host which can provide authentication and buffering. The split-slot arrangement might be viewed as a new protocol level which operates somewhere between the currently defined MTS-MTS and UA-UA levels
