I've used Thunderbird for about a year, & seek something better.
Thunderbird behaves too strangely.
Example: There'll be a list of new in-mail. If you click out of the in-box to a different box, then back to the in-box, those e-mails are "gone." Maybe 2 or 3 days later at a randon time, those e-mails pop back. For a random (long) time that e-mail was "lost."
Example: Say maybe 3 or 4 times you click to check for new mail, it'll go do it. Then like a stubborn mule next time it refuses to check, no matter what you try. Only way to check for new mail again is to close & reopen Thunderbird. Of course when you reopen, then e-mails already in the box might become "lost."
Example: I've never been able to find where Thunderbird stores mail on my hard drive. I've tried sending myself messages with words in the subject and body that I sent Windows on searches for. Windows can't find it. I can't find either it by slow visual hunting through the root structure. Mozilla Thunderbird FAQ claims mail resides at C:\WINDOWS\Application Data\Thunderbird\Profiles\ptcqba3c.default\Mail, and that file structure is indeed on my C:. But the e-mails are not in there. Weird.
If I could just locate it, I could open e-mail with Notepad to workaround the troubling times when e-mail disappears from Thunderbird. But I can't find it. When it's "lost" it's gone off Earth.
Example: Sometimes the inbox shows e-mails listed there, and even lists new ones as they arrive, but Thunderbird won't let you open them or delete them. Only way is to close the program, & just hope they don't get "lost" when you reopen Thunderbird.
Example: Thunderbird randomly disconnects my dial-up modem. That never happened with e-mail clients I previously used, like Neoplanet and Outlook Express and Eudoramail.
If you've experienced an e-mail client you like, please post about it here.