I have been using ThuderBird on my USB fash drive and it has worked great. However, on my computer I run POPFile -- an automatic mail classification tool. It would be great if POPFile was made portable.
You can check out popfile at popfile.sf.net.
Thanks, Evan
POPFile (see the POPFile forums at sourceforge.net) is portable, you should be able to copy the POPFile directory to your portable drive, and run it from there. Just a warning, though, it will run differently on the computer the installer was run from when compared to the portable version. I sync the files regularly, and make sure that the version of POPFile installed on the computer (versus portable drive) is not running (of course this is pretty much a prerequisite for effective synchronization).
Also, clean up the history before sync. I found it gets messy quickly with all the saved messages, especially if different installations have different histories. If you can select which files to synchronize, choose popfile.cfg, popfile.db, and popfile.db.backup.