You must sign into a mailbox with Apple Mail to change the default Mac mail app

&gt; You might be prompted to add an email account before you can change the setting in Mail. If you don&#x27;t want to do that, check the preferences of the other email app. You might be able to set a default email app from there.<p>It says this, but Microsoft Outlook for Mac does not offer that setting. Apple Mail hides the &#x27;Preferences&#x27; until you sign in to a mailbox, in which case it immediately downloads your email

I think example.com runs a generic mail server that will accept any login - at least when I was playing around with implementing something like that, my naive test account @example.com found settings and got added successfully to multiple mail clients. Obviously no mail will ever arrive there, but it might be a good workaround for this.