How to change the "indicator" default setting, like mail agent

There is no setting option in gconf-editor or panel properties.
But you can just use the file to change the preferred application.

- In /usr/share/indicators/messages/applications/ there are some files, like empathy/evolution/gwibber in my folder by default.
- You can add a "thunderbird" file.
- And add a line in it as "/usr/share/applications/thunderbird.desktop".
- Then~~~you will see the indicator show you "Mozilla Thunderbird" dynamically.