Finally there is any certainty that mozilla thunderbird will become Ubuntu 11.10 Oneric Ocelot default mail application replacing Evolution, this news also written by Mike Conley (Mozilla Thunderbird Staf) on his blog here.

Thunderbird will be the default e-mail client in Ubuntu Oneiric
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Well, kinda, yeah.  But the thing is, those first alphas were just to get a sense of how Thunderbird would work as the default client, and to gather feedback.  At a moments notice, Canonical could have backed it out and switched it back to Evolution.
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But they’ve given their thumbs up, and they’re fully on board.

Ok guys, See you on next october