I'm running Sparrow mail client and would love to disable Mail.app, which keeps opening from time to time.
From Apple Support, the steps to switch to a different mail reader are:
- Open Mail (/Applications/Mail)
- From the Mail menu, choose Preferences
- Click the General button
- Choose your desired default email application from the "Default email reader" pop-up menu.
This article also provides instructions to change the default mail app by editing the appropriate plist configuration.