A few hours after Thunderbird decided that it really MUST download all the messages all over again for my Yahoo account. It has now decided that it must do so AGAIN! Why?! Was the thrill of downloading 16000 emails not enough the first time round? Must it do so again? And how often will I find my email box bulging with more duplicates of 4 year old emails?
The Blog of Colin Mackay 
Why is a Microsoft MVP using Thunderbird and not Outlook?!? Off with his head I say :-).Are you sure it is a Thunderbird issue? Could it possibly be an issue on the Yaah-booo side?
Actually, I use both. I use Thunderbird for my yahoo accounts. I use outlook for my ScottishDevelopers email.It is possibly an issue on the Yahoo side. I must admit I didn’t look too much in to it. However, since it downloaded all the emails once again after I’d deleted all the duplicates I suspect Thunderbird isn’t telling Yahoo where it got to properly. It also happened very shortly after Thunderbird upgraded itself, so it is possible that a new bug got introduced.
I doesn’t know much about that but i heard that thunderbird is unfit, though every time it asks us to login to open the mailbox, there’s no option to save an email into the Outbox, there’s no way to set fonts from within an email.
Thunderbird is far better than anything MS bring out or try to bring out. Outlook has so many bugs it makes the amazon look bare.