Let me also put in a commercial for Courier MTA, which has this
big-brother feature out of the box (among others :).
http://www.courier-mta.org
# This is the big-brother option that saves a copy of EACH and EVERY
# message passing through the system. Uncomment ARCHIVEDIR, and after
# a message is delivered, its queue and data file is moved to ARCHIVEDIR
# instead of being deleted. You must create the ARCHIVEDIR directory
# yourself, and it must be owned by the "courier" userid.
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