Thunderbird is typical of Mozilla design: decent enough for simple usage but put to the test it bugs out.

With daily use over the past few months I find that the program requires to be forced closed on a daily basis to eleviate phantom windows. The address book could take a cue from Outlook, the search feature could take a cue from Eudora which is superfast, and the email address auto-fill is often unpredictable.

So far, Thunderbird is a turkey, but its potential can go the way of Firefox (to browsers) if the delvelopers are smart about it.