This subject has been debated before:
si.Mail (open source free mail client)
http://kmeleonbrowser.org/forum/read.php?1,77051,77051#msg-77051
Pegasus Mail (the oldest, but still going strong)
Pmail.com
Other choices (including MSOE)
http://kmeleonbrowser.org/forum/read.php?1,23886,24168#msg-24168
Some help on SPAM prevention
http://kmeleonbrowser.org/forum/read.php?1,32031,32088#m
Forum:
General