In Windows Explorer click Tools | Folder Options | File Types. Scroll
down a bit and find URL: MailTo Protocol among those with no extension.
Select it and click Advanced. Select 'open' and click Edit. Make sure
Application points to Outlook.

When I do the above setup for MailTo, when I click on a reply to email
address or try to send a link, Outlook opens continuously where over 50
instances initiate and it is rough to stop them continuously opening. I
selected the Run option with the Outlook program to be set up under the
Advanced selection.