Oh heck! I was kinda shocked. I put in a support ticket regarding the emails not received in Vivaldi Mail, and of course I got an email letting me know I have a ticket in. Well, a few minutes later I glance up at the time and notice that the 'indicator applet' for the Mate Desktop is 'lit'. I'm thinking dear God, I just updated Firefox yesterday, there's another one? Au contraire! Vivaldi's email client works with it
That was a pleasant surprise.