Today at the Ubuntu Developer Summit, there was a discussion on which application should be shipped as the default email client with Ubuntu 11.10. So far Ubuntu has been shipping with Evolution as the default email client. Of late, there has been a growing number of people who are not pleased with Evolution and instead asking for Ubuntu to ship with Thunderbird. So, basically the discussion was to decide between Evolution and Thunderbird.
Being a part of GNOME, Evolution integrates very well with the GNOME desktop. But it suffers from several flaws such as an outdated and confusing user interface. Evolution is also fairly slow compared to other email clients.
Thunderbird has a fairly modern tab-based interface. It is quite fast compared to Evolution and has a lot of extensions to extend its functionalities. However, Thunderbird still has a few critical problems. Thunderbird does not have calender support (although there is an extension for that) and does not integrate well with the GNOME desktop. Another problem for Thunderbird is that it does not work with Microsoft Exchange. Microsoft Exchange support is particularly important as a lot of businesses uses it.
Because of these shortcomings of Thunderbird, it has been decided that Thunderbird will become the default email client in Ubuntu 11.10 if these issues are fixed. For now though, development for Ubuntu 11.10 will go ahead with Evolution.
However, work is already going on with Thunderbird to address these issues. Thunderbird is being integrated into Unity and it is getting contact sync with Ubuntu One as well.
You can view the notes from the discussion here.