Microsoft has been working on a completely new Outlook mail app for Windows, which looks like quite the improvement. In the meantime, a change is rolling out that definitely isn't an improvement.
Microsoft announced an upcoming to change to Outlook in the Microsoft 365 admin center. Outlook for Windows will force web links in emails to open in Microsoft Edge -- regardless of the system's default browser. The same behavior is planned for links in Microsoft Teams "in the future." Microsoft told The Verge in a statement that there will be a setting to turn off the behavior.
Microsoft's reasoning for the change is "to help increase productivity while working online," because there will also be limited integration with Edge's sidebar. For example, if you open a link in an email, Edge will open the link with the email visible in the browser's sidebar. Of course, you could just switch back to Outlook or Teams to see the original message again, or use the window snapping system in Windows to display both applications at once. Neither of those options helps boost Edge's browser market share, though, which clearly matters a lot to Microsoft right now.
The company said in a message to IT admins, "Web links from emails in the Outlook for windows app will open side-by-side with the email in Microsoft Edge so users can easily reference the link and email without switching back and forth between apps. The email will open in the Outlook app in the Edge sidebar. Links will open in Microsoft Edge even if it is not the system default browser in Windows."
The change only affects Outlook for Windows, with the Mac, mobile, and web versions currently unaffected. However, at this rate, Microsoft might update those versions with the same functionality. It's just another change intended to boost usage of Edge, which in turn serves as a gateway to other Microsoft services (like Bing Chat). As Captain Barbosa would say, the default browser settings are apparently "more of what you call guidelines than actual rules."