Change default mail client [duplicate]
Solution 1:
Hi along with changing in Default application list, you need to some more work.You need to install this package to arrange the room in Notification area.
Open your terminal with ctrl+alt+T and type this
sudo apt-get install evolution-indicator
Then Re-login/Restart to apply the changes.
PS:If you don't want to use Thunderbird , then you'd better uninstall it.