Cannot Set Default Mail App in El Capitan

For anyone still having this problem I have an easy solution which, I think, goes to the root of the problem.

Go into system preferences and click on iCloud. Unclick Mail. Close out of system preferences. Now go to Mail and select Outlook for your default. Just to be safe I then closed all programs (Mail last) and rebooted.

That solved it for me! Hope it works for you.

After you select Outlook in the Mail preference, go to setting and select internet account than select your account on the right and on the left you will see USE With: make sure Mail is unchecked and everything will work the way you want it.

I was successfully able to change the default mail reader by going to Apple Mail --> [Preferences]. Under [Genera], do NOT select the application you want to switch to from the drop-down. Click [Select] from the drop-down options and browse for the appropriate mail application you want to use. Select it from here. Close/quit the Apple Mail application. Re-open Apple Mail application and you will see that your selection has been saved.

For me it was helpful to upgrade my mac to Sierra OS. After that I can make Outlook as a default mail app.

This worked for me! thank you.

This is the easiest solution on the thread and it worked perfectly.