In a recent update, Microsoft gave Outlook its own Default Web Browser setting independent of your Windows setting. First, lets make sure Windows is set to what you want.
Click the spyglass icon in the bottom-left corner of your screen to open the Search function:
In that Search box, type “default” to search for and click the Default Apps settings:
In the settings screen that pops up, scroll down to Web Browser and make sure it’s set to whichever you prefer:
Now that that’s sorted out, let’s open Outlook and check that setting.
Click the “File” button in the top-left to get to the File Menu:
In that screen, click the “Options” button in the bottom-left to open the Outlook Options pop-up:
Once that opens, click “Advanced” to go to the Advanced Settings:
In the Advanced Settings you should see a section titled “File and browser preferences”:
Switch that from Microsoft Edge to Default Browser and now Outlook will use your Windows Default Browser when opening links in emails.