How can I set Chrome to let mailto: links be handled by Outlook.com?

It's a bit of a convoluted process, but you can do it using navigator.registerProtocolHandler and a web site that rewrites the URL's query string a little.

Go to http://terrific-charger.glitch.me. This is a tiny Glitch project I wrote for which the code is below, but you can verify there is no hanky panky here: https://glitch.com/edit/#!/terrific-charger?path=script.js:1:0.

Open the Chrome console (ctrl+shift+j), then paste the following code into it and hit enter:

navigator.registerProtocolHandler('mailto','http://terrific-charger.glitch.me/?mailto=%s','Outlook.com');

Chrome will now tell you that "http://terrific-charger.glitch.me/ wants to Open email links". Click "Allow". That's it!

To undo the new behavior, go to: chrome://settings/handlers, and remove the listed handler using the three-dot menu.

Glitch code:

if (location.search.length)
   location = 'https://outlook.live.com/default.aspx?rru=compose&to=' + 
       decodeURIComponent(location.search).slice('?mailto=mailto:'.length).replace('?','&');

My comapny has Office 365 and the online Outlook is hosted at https://outlook.office.com/. To set this site as e-mail handler in Chrome:

1. Open the web mail site
2. Open the Chrome console (ctrl+shift+j)
3. Copy the following line in the console and hit Enter:

navigator.registerProtocolHandler("mailto", "https://outlook.office.com/?path=/mail/action/compose&to=%s", "Office 365");

If you change your mind open chrome://settings/handlers and remove the listed handler using the three-dot menu.