I am still enjoying a love/hate relationship with Apple Mail. (See Mac E-mail Again, 08 Mar 2006.) I'ved not lost any email, but there are many times that I've deleted or foldered e-mail, only to have it turn up again in my Inbox. Grrrrr.
Then I noticed others in Apple's discussion forums complaining. (For example, see Mail repeatedly downloading items.) Ther have been problems with Apple's own .Mac site as well as BT Yahoo.
I decided to try Thunderbird again. It did not misbehave! But, for some reason I couldn't save sent mail to a Sent mailbox. Grrr again. And Mail has other features I mentioned in the above-cited entry. So, I wait and hope for a fix now that it is more than just me.