I used Thunderbird email client, by Mozilla who also makes Firefox, until they released version 2. With that version they introduced a new bug, namely that when you have it set not to retreive emails from the server each time it is started, it does so anyway. For reasons that don't matter that is not acceptable to me, and I had to search for a different client to use, and settled on "Claws" which I don't like but is the "least bad" one that I found. Mozilla is aware of the bug, but never fixed it in version 2.
I noticed that they have released Version 3, so I gave it a try, only to find that they have not fixed the bug in that version either. Mozilla used to make good stuff, but I'm fairly unimpressed with the latest iterations of Firefox, and its Thunderbird email client is untrustworthy.