Linux offers an abundance of email client options. Most new users simply go with the default client of the distribution they are using. GNOME-based distributions typically use Evolution, while KDE uses KMail. But many alternative clients are available.
Mozilla Thunderbird is probably the most popular alternative to the GNOME/KDE incumbents. The latest versions of Thunderbird offers Web 2.0 features like tabbing, advanced history searching, and spam deletion. Another client, Claws, on the other hand, is more about performance. In addition to speed, it offers stability and intuitive use.
All of the preceding alternatives run from a graphical user interface, but there are command-line clients too, some of which are just as full-featured as their GUI equivalents. Mutt, for instance, includes support for message threading, full MIME attachment support including PGP, reply-to-list for mailing lists, POP3, IMAP, several mailbox formats, and dozens of other functions too numerous to mention. Alpine is another command-line email client.
Because they're all free, you can try all of these email clients until you find one that's right for you.