Here is the steps to change default mail app in OS X:
- Open Mail app.
- Select Preferences -> General. Under "Default email reader", select your preferred mail app.
I am running into a problem by following the steps: the default email reader changes back to the original mail app seconds after I made the change. Here is the solution that works for me. Open terminal. Copy and paste the following script.
/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -all local,system,user
Then try step 1-2 again, and it won't change back again!