There is a common MacOS Sierra problem when setting the default mail client in the Mail Preferences screen where the setting will revert away from the desired setting. This creates a situation where you cannot set the default client as desired.
To solve this problem, you will need to reset the LaunchServices database. Be aware that these steps this will set all of your default apps back to their original settings.
- From the Finder, select “Go” > “Utilities“
- Launch the “Terminal” application.
- Type the following, then press “Return/Enter“:
/System/Library/Frameworks/CoreServices.framework/Versions/A/Frameworks/LaunchServices.framework/Versions/A/Support/lsregister -kill -r -all local,system,user
Close out of Terminal and try to set the default mail application again. You should be able to without issue.
From the Finder, go to “Apple Menu” > “System Preferences…” > “iCloud” > “Sign Out“.