Mozilla's Thunderbird mail reader takes a bit of work to configure properly for your Athena account. To use it you will need to open the Mail & Newsgroups window and follow the dialog to fill in your account information. You will need to have your Mozilla certificates already working when you first try to read your mail, and you will need to know the name of your IMAP server, which you can find out by typing the command chpobox.
Gnus is an emacs-based mail and news reader. It is best not used for reading mail, but it is highly useful as a news reader. To start it, open an emacs window and type M-x gnus. One other client available is mutt.