> Is it possible to truly verify that a message has come from a specified user?
Check the PGP signature ?
> We would like the postmaster to be the only user able to access certain site-
> wide lists, but need to be sure that the sender really is the postmaster.
A technique we have recently implemented is a "poor man's moderation".
It may be suitable for you in this case ...
Using a system-wide filter, freeze all mesages to the list as they arrive.
Postmaster presumably can thaw the message, and if others cannot, you have a
tedious (but usable) mechanism -- Ok for low traffic lists.
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