https://www.theregister.co.uk/2018/03/18/windows_10_tests_forcing_use_of_edge_browser_for_links_in_windows_mail/

Microsoft’s about to test a new feature of Windows 10 that will force users to employ its Edge browser under some circumstances.

Revealed in the announcement of Windows 10 Insider Preview Build 17623 for Skip Ahead, the change means “we will begin testing a change where links clicked on within the Windows Mail app will open in Microsoft Edge”.