Thank you operaoperandi! I went into Mail --> Preferences --> General and found a "Default email reader" of Opera.app. I didn't put that there! I never would. Nor do I remember answering a question when starting Opera that granted it permission to make that change (though I won't swear that it's impossible -- I sometimes read things a bit too quickly).

I really appreciate your help.

It's been more than two days since I posted that. Does no one have any insight into how to resolve this?

I am running MacOS Sierra 10.12.1 and I don't use web-based email. I use a standalone email client that sends/receives through a SMTP/POP3 server that I host locally.

When I clicked an email MAILTO: link in Opera 41, it opened up a gmail form. Under Preferences --> Privacy & Security --> Handlers, there was a handler for MAILTO: links to send them to gmail. After deleting that, MAILTO: links did nothing.

I found the MAILTO: extension and installed that, but it only lets me select web-based email, which is fine for kids and grandparents, but not for me.

So, how do I get Opera to use an external application (the system default one, specifically) for email?