Step 1: Hit Windows + W to open the Settings Search screen. Type Default Programs in the search box and from the result set, choose Default Programs.

Step 2: On the next window click on the link “Associate a file type or a protocol with a program.” 

Step 3: Scroll down the programs list to find the “Protocols” section. Then look for “MAILTO” and double click on it to set your preference.

Step 4: You can then choose the appropriate program from the list.

Step 5: Choose the one you wish to set as your default.

Step 6: Back on the Set Associations window you may want to verify if things have been set accordingly. Check the Current Default against MAILTO option.