I set up my parents on Pegasus mail, and they have been happy with that. I do not know if it will read Netscape formed folders, but I'm sure there is some kind of conversion utility out there if it doesn't.

Although I will have to echo the comment that Mozilla 1.3 and later has been very stable for me as an e-mail client. The spam filters work well, especially when used with SpamAssasin. One or the other catches over 99% of my spam, and I get over 100 pieces a day.