What this discussion really points out is the need in mail packages
for relational capability. At one time I built a document
management system (in essence) for mail for the NeXT, called
MailQuery. It used Thunderstone's Metamorph full text search engine
(now part of Texis). With relational capability, we're no longer
limited to a single tree structure of folders based on (whatever) -
we can have 'folders' and 'trees' based on subject, author, date,
concepts expressed, or whatever.
I don't know Pegasus, but I use Eudora - many folks find it the
most useful of the Wysiwyg-type mail clients. I don't like any of
the mailers I've used on Unix, though I use Procmail extensively for
varous things. I hear that Qualcomm is trying to figure out what
to do with Eudora - OPEN SOURCE it!!