Red Hat-Mozilla Mail/Thunderbird
Thunderbird provides a similar interface to Outlook Express. It has excellent support for IMAP type mail, spam filtering and comes with encryption support in the form of a plugin.
If you install Thunderbird under Redhat Fedora, you would be best off getting the RPM file to install it from fedora.us, as this version is slightly modified to integrate well with Fedora.
There is a really great plugin that works with The GNU Privacy Guard that allows encryption and decryption of messages. Anyone who uses email to send and recieve sensitive messages should really take this into consideration. Anyone who wants to use this feature must have GnuPG installed.