I have been happily using Thunderbird to deal with my email for the past year or so. However, a couple of weeks ago I discovered that it sometimes loses emails when I move them from one folder to another. A bit of investigation with a text editor revealed that the emails are, in fact, still there. The problem seems to be that Thunderbird's indexing system has lost track of them.
That would be no more than a nuisance if I were simply dealing with personal emails, but a lot of my work comes to me via email so reliability is essential. Clearly Thunderbird is not absolutely reliable, so it had to go.
I really don't have time to learn how to use a completely new email program, so my choice of alternatives was limited to those I have used in the past: Outlook, Outlook Express and Opera. Since I had several thousand emails to export from Thunderbird, Outlook and Outlook Express ruled themselves out by having very inadequate import facilities.
So Thunderbird is gone, and Opera is go. The fact that it also offers an extremely fast web browser, a newsreader, IRC client and BitTorrent client makes Opera almost irresistible. My only niggle is that it is not very good at displaying web pages that have been sloppily coded (i.e. optimized for Internet Explorer).