Mozilla recently patched and updated open-source e-mail program Thunderbird, releasing version 1.5.0.12, which improves stability and closes security holes, one of them critical.
The Mozilla announcement says that Thunderbird 1.5.0.x users will get an automatic upgrade prompt over the next 24 to 48 hours. The update is also available from the Thunderbird archives.
If you're still using Thunderbird 1.5, you'll want to apply this update. You might also want to try Thunderbird 2, released in April.