What does the default mail app do? It opens the mail app whenever you click on a mail-to email link on the web page. Out of the box, the default mail app is the Mail app that comes with Windows 10. But if you are using Microsoft Outlook, you do want to change that default mail app to Outlook, instead of the Mail app that is never configured for your email account.
The step is relatively simple. Click Start and the gear icon to open the Settings app, go to Apps and Default apps, and set Outlook as the Email default app there.
It stays this way until the next time your Windows 10 gets another big feature update. Then, you will need to do it all over again.
It could be annoying and tedious if you manage hundreds of computers. A Group Policy implementation would be much easier.
First, export the current app associations to an XML file. Run the following command in an elevated command prompt window.
DISM /Online /Export-DefaultAppAssociations:"d:\AppAssociations.xml"
To make sure that Outlook is the default mail app for opening mail-to links, check and see if you can find a line like below in the XML file you exported.
<Association ApplicationName="Outlook" ProgId="Outlook.URL.mailto.15" Identifier="mailto" />
Then, open Group Policy Editor, go to the following
Computer Configuration > Administrative Templates > Windows Components > File Explorer
And double-click the Set a default associations configuration file policy and set it to Enable, with the path to the app association XML file you just exported.
That should do it. The default app association will take place next time when the user signs in again.
If Group Policy is out of the question, this command line should be the next option to try.
SetUserFTA.exe mailto Outlook.URL.mailto.15
Put the command line into the login script and you are all set.