For me, the defining characteristic had to be that it "plays well" with
Unix. That means it can't have a policy of downloading all emails to the
local machine (as early version of Outlook did), and it must be able to
save fcc's to an IMAP folder (as current versions of Outlook can't do).
Despite the dire warnings, I *do* use a Mozilla nightly build to read
email on my laptop. I like having the knowledge that in a pinch I can
pop my floppy with PuTTY on it into a random computer, log into my
server, and run pine with the environment I expect.
Now if only Mozilla had "roles", I wouldn't need to run pine on a daily
basis to read mailing lists. :)
David