By default, kubuntu uses KMail as the default email client. Since I installed the KDE desktop on top of my Ubuntu installation, I wanted to keep Evolution as my default email client. I used the following steps to make this happen, but they will work for changing the default client to whichever email application you have installed.
1. Click the K Menu.
2. Select System Settings.
3. Under Personal, select Default Applications.
4. In the left pane, select Email Client.
5. In the right pane, select the Use a different email client radio button
6. Click the button at the end of the textbox.
7. Browse to the desired email application. In my case, I expanded Internet and selected Evolution Mail.
8. Click OK.
9. Click Apply.
10. Close the Default Applications window.