It's your OS that does this, not InDesign.
MY OS uses Mozilla Mail, and I've never used MS Outlook, which is why I
don't understand how InDesign leads me there? How do I tell Windows not
to point me to Outlook?
Control Panel > Default Programs, and it may be near impossible to fix as Windows seems reluctant, in my experience to assign programs to protocols properly.
Do you mean you use Thunderbird ( the mozilla email client), or some add-on in Firefox fore webmail access? I use the old Windows Mail from Vista (though a hack in Win 7), and I had one heck of a time getting Widows to associate it with the mailto: protocol after it was already associated with Windows Live Mail.