Evolution is the default email client used in Ubuntu. Its not a bad client, but I'm surprised it was chosen over Thunderbird 3.0.
I believe the decision was based on the fact that Evolution supports GPG. If I understand correctly, Thunderbird only supports GPG via a plugin, which is no longer maintained - enigmail.
Evolution is close to being as good as Thunderbird, in my opinion. If it supported custom "Sent" folders, that would be a step in the right direction!
SMTP uses port 25 by default, but many ISPs block port 25 in a lame attempt to limit spam. Therefore, many email users must use an alternate port, like port 587.