Its interesting. I’d tried to share a post by email, both of my browsers (Chrome and Firefox last versions) are giving a new completely empty browsers windows. (Firefox is opening a new tab and new window at the same time, and both empty!)
My default email program is MS-Office-2016-Outlook, if i right-click on any file in my Win10 system, → Send-to → email recipient, it will open an Outlook window.
I’ll try to setup my browsers for that if possible, but the main problem will be at the community user’s side. Many different OS’s, browsers and even the mobile cases! Would be so technical and hard to explain!
Is there is a way for Discourse to check if there is a system default email client(web or local) from Discourse’s share by email button?
Edit:
I tried to setup Chrome but there is no way to add handler!
But I noticed this default choice, Allow site to ask… Dos this give a chance to Discourse fix the problem?
I did click from the first screenshot of Learn more, and I found the follwings:
Handlers. Chrome allows web services to ask if you’d like to use them to open certain links. For example, certain links can open an email program. If you're using Gmail or Hotmail, this setting lets the site handle links that would otherwise open a program outside your browser.
So we may add it at the user Preferences as a field to let users enter/chose their preferred default method of email client. Could be?

