If I hit "submit" on a form and I saw it start to open a new Gmail tab in my browser, I'm going to close the new Gmail tab before it even has time to finish loading. (Or same if I saw it opening Mail.app.)
I'd just assume the site was trying to trigger some kind of spam e-mail or something.
The idea that I'd fill out a form on a site, then submitting it would open my mail program, and I'd then have to hit send there, and then close my mail tab/window (not to mention exposing my e-mail address to the site when maybe I wouldn't want to), is some of the worst UX I've ever heard of.
I have a Pavlovian annoyance response to noticing that I have inadvertently clicked a mailto link, because back in ~2005 firefox would try to start Evolution. I usually only noticed the click because of the sound of my spinning disk thrashing to try to lift into memory hundreds of MB of dependencies from their rust platter slumber. Evolution generally didn't even load enough to so much as show its splash screen before I found a terminal and killed the process tree.
I'd just assume the site was trying to trigger some kind of spam e-mail or something.
The idea that I'd fill out a form on a site, then submitting it would open my mail program, and I'd then have to hit send there, and then close my mail tab/window (not to mention exposing my e-mail address to the site when maybe I wouldn't want to), is some of the worst UX I've ever heard of.