An MUA normally only interacts with its local MTA. MTAs are frequently large and complex, and normally each system only uses one MTA. MUAs can vary in complexity and each system may have more than one (in our case many). So, for example, MMDF and sendmail are MTAs, while elm, ream and MH are MUAs.
We use MMDF (Multi-Channel Memo Distribution Facility) as our MTA and this document describes our MMDF setup. We're currently running MMDF-IIb Patch Level 43; on the Suns, we have a few additional patches from IEunet. While MMDF is not that new and much recent development has gone into competing systems, MMDF is remarkably stable and well behaved. It also appears to be secure.
Up to top level, on to System Description.