Try kmail, comes with KDE, very outlook like, easy to configure and supports multiple accounts. The only drawback I found, at least with the release I'm using, is that you send through the same SMTP server for all your accounts. It's a quick reconfiguration if you want to use another SMTP server though, or you can use sendmail instead of a host's SMTP server.
Most of the Linux distros come with at least 4 or 5 e-mail clients, so you aren't stuck with using navigator if you don't want to.
I like kmail too -- it is simple and functional. If you like the function of Netscape but want more eye candy + a bit more features try Mozilla's. Evolution is a new Gnome mail client that is also has very elaborate organizer features, like a searchable contact list, and calendar.
I just started using Sylpheed [sylpheed.good-day.net], so far I like it a lot. It supports multiple accounts, and several protocols including IMAP. It is also a light weight compared to kmail. The one thing it is missing is a spell check.