A recent change will now default hyperlinks in Outlook to open in Microsoft Edge regardless of your default browser choice set in Windows.
Multitask smarter with Microsoft 365 and Edge | Microsoft 365 Blog
For Windows 10:
To resolve the issue, change the default browser in Outlook options.
- Open Default Apps in Settings and set the default browser.
- Launch Microsoft Outlook, and click File → Options → Advanced.
- Under “Link handling”, change the dropdown option for “Open hyperlinks from Outlook in:” to “Default Browser“.
- Click OK.
That’s it. Outlook should now open hyperlinks in your default browser instead of Edge.