This is the output of my 'beagle-info --list-backends'
Code:
KMail (/usr/lib/beagle/BeagleDaemonLib.dll)
Files (/usr/lib/beagle/BeagleDaemonLib.dll)
GaimLog (/usr/lib/beagle/BeagleDaemonLib.dll)
IndexingService (/usr/lib/beagle/BeagleDaemonLib.dll)
Tomboy (/usr/lib/beagle/BeagleDaemonLib.dll)
Blam (/usr/lib/beagle/BeagleDaemonLib.dll)
Liferea (/usr/lib/beagle/BeagleDaemonLib.dll)
Akregator (/usr/lib/beagle/BeagleDaemonLib.dll)
KonquerorHistory (/usr/lib/beagle/BeagleDaemonLib.dll)
Kopete (/usr/lib/beagle/BeagleDaemonLib.dll)
As you can see I have no thunderbird backend, so it must be only on edgy (I can see by your profile that you have edgy, and I have dapper).
As for Evolution, I know beagle indexes it automatically, but unfortunately I do not like it. I just need a plain and simple email client that's powerful just at it and nothing else. I also like the fact that Thunderbird is multi-platform (I know there's a beta Evolution for windows, but it's not ready to use)