Found out this afternoon was Mr Shuttleworth uses for email...
Thunderbird - so why is evolution the default?

Evolution is the default because it is part of GNOME, the desktop environment
used by Ubuntu. The Canonical philosophy is not to re-invent the wheel, so as
far as possible they use KDE and GNOME as it is out of the box, adding as
necessary, but not removing.

The release schedule of Ubuntu is timed to coincide with GNOME updates. That is
how integral it is to Ubuntu.