I just successfully changed it, but it was a janky experience. Until I added an account to the mail app, the preference used to change the default email reader remained greyed out because I was being prompted to add an email account. Only once I added a fake account did the preference option become available and let me change the default mailto handler. Removing the fake email account caused the preference to become greyed out (but it retained Outlook as the default).

Here's another example: Try and change the right click -> search with google menu option to 1) not open in Safari and 2) search using another search engine.