I know this isn't useful info, but Outlook for Mac is crap. It's crap coated crap with crap filling. It has great features, but it's poorly implemented. It's also missing features that are present in the Windows version. All this said, some of my clients actually like using it, so Outlook has been a thorn in my side for years. I have never been able to set it as the default mail app using a script. Somehow, it always stops working. Microsoft released an app that you can use to set Outlook as the default mail app, so I distribute it via Self Service for users who want to set Outlook as the default. Soon, I will be moving one of my clients back to Apple Mail, Calendar, Contacts, Reminders, and Notes because all of the employees hate Outlook. Searching breaks a lot. It's slow to launch compared to Apple Mail, and it often has search issues that can only be solved by completely removing the users's account and setting it back up again. Outlook for Mac is not a business class app. Microsoft is screwing over every Apple customer by selling them Office for Mac for the same price that Windows users pay, but Mac users get less of a product. I barely use Office on either of my Macs. When I write something to be sent to others for editing, I will use Word, Excel, etc., but if it doesn't need to be edited by someone else, I write everything using Pages and Numbers. I output my work to PDF. I'm sorry for the rant, but we should not have to even ask how to set Outlook as the default mail, calendar, and contacts app. It should be easier than this. While we can blame Apple for this too, Microsoft should be making it easier, and they should make a product worth going through this much trouble. Microsoft should be working harder to make a great product for Mac users. Apple got Microsoft into the application software business, so I think that they should show Mac users some gratitude.

The PKG suggested by @dubel works perfectly! Works instantly, no reboot required.

As the name of the pkg suggests “MailToOtlook.pkg” sets Outlook as the default mail app. Unfortunately it does not set it as the default calendar app.
Anyone have any suggestions for that?

A while back Rich Trouton put out an Automator-based app that uses duti to set Outlook as default for mail, calendar & contacts. I use the app with my fleet by deploying it to each machine, then scoping a policy to all machines with Outlook installed to launch it from Self Service.

Keep in mind that these settings are user-specific so have to run from the user context & not as root. This is why it’s set for the user to launch the app to apply settings rather than pushing a policy to do so.

with Python 2.x being removed (and Python 3 no longer pre-installed) in macOS 12.3 or later I highly recommend using MailToOutlook 2.0 via: https://macadmins.software/tools/