Old Outlook now defaults to opening links in the Microsoft Edge internet browser. Follow these steps to revert your internet browser, in Outlook, from Edge back to the default browser that you selected in Settings > Apps > Default apps. Note that new Outlook defaults to the default browser that you selected in system settings.

1. Open Outlook and navigate to File > Options > Advanced > select Default Browser from the dropdown under Link Handling.
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2. Close out of Outlook entirely and then reopen the application.
3. You should now be able to open links from Outlook in your default browser.