As a subscriber to Office 365, I thought I’d give the entire Office 365 Suite a shot on my Macbook Pro. While I’m pretty happy with everything on offer, Outlook just never quite meshed with my reliance on Calendar, Reminders, and Contacts bundled with OS X and iOS. So after switching back, I’ve put up with Outlook always launching if I happened to click on an email link to start an email composition. This impediment isn’t due to me not changing the default mail app on a few occasions, no, it’s from the setting not saving.
That brings me to today. Instead of simply going to the Mail > Preferences > General and changing the dropdown (Default email reader) from Outlook to Mail only to have the setting change back immediately, this time, I chose ‘Select…’ and browsed for Mail in the Applications folder. I closed and re-opened Mail, and the preference stayed. This process should also work for anyone trying to do the opposite (that is, change the default from Mail to Outlook.) If I had to guess why this happens, it’s possible that as a result of a major Mail release, the location of some dependency or a symlink became broken. That’s just a guess. I hope that helps someone!