Did you check if is there enabled the Outlook Add-In option in the menu extras > options > advanced? IF yes then disable it, reboot and start again.
For the Windows side, assuming that you are using Windows 10, this guide can help you change the default applications of Windows.
Windows 10 yes. Default apps in windows set correctly to my preferred email client.
I cannot find a Outlook add-in option in advanced options?
one possible solution is to uninstall it, reboot your pc and install it again?
Remember also to remove the settings during uninstall process by checking the related box, in order to delete any bad settings.
Good advice. Didnt work.
I guess its a windows thing then. Trying to lookup the what apps use wich apps.
Trying to remove outlook too.
Please wait, I just found this related post with screenshots which worked for others.
Please give it a look!
It is good news if is working now!
Just one notice please, you could also rename the folder instead of deleting it, just in case in future something needed from this folder.