The default mail program is the software that is registered with your operating system as the software that is to handle mailto URL's. When another application wants to send an email, it looks up the default and passes the email to the default mail program.
Use the following steps to change the default mail program in Mac OS X:
Open the Mail application from the Applications folder.
Click on the Mail menu and select Preferences...
Click the General button. If you don't see the General button, click on the small oval in the upper right corner to reveal it.
Select a new default mail program from the Default Email Reader menu (e.g. Thunderbird, Entourage).