By default, whether you have the Outlook app installed or not, the default MailTo app is the Mail app that comes with Windows. If that’s not what you want, here is how to change it.
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.
Through Group Policy
If you manage hundreds of computers, implementing it through Group Policy is the way to go.
First, if you have never set it up before, you will need a sample XML first to start with. Run the following command in an elevated command prompt window.
DISM /Online /Export-DefaultAppAssociations:"d:\AppAssociations.xml"
To make sure that Outlook should be the default MailTo app for opening any mail-to links on the website, copy and paste the following line into the XML file first.
<Association ApplicationName="Outlook" ProgId="Outlook.URL.mailto.15" Identifier="mailto" />
Then, open Group Policy Editor, go to the following location,
Computer Configuration > Administrative Templates > Windows Components > File Explorer
And enable the Set a default associations configuration file policy with the path to the app association XML file you just prepared.
The default app association will take place next time when users log into their computers.
If Group Policy is out of the question, this command line should be the next best option to try.
SetUserFTA.exe mailto Outlook.URL.mailto.15
Merge the command line into your login script and you are all set.