I'm gonna go ahead and say MOST of the marketplace bits should be cross version, you just need assembly binding in your webconfig.
The only cases in which they'd fail are if like a control is using a feature or API call that's not available in an older version, stuff like that.
...you can only test though
I know I've had to make MINIMAL (if any) changes on my marketplace controls. Has been stable since about 5.0.
If you're on 6 you should already have forms notifications. It's not like that widget...but kinda the same idea....and there's certainly nothing stopping you from using that control idea to send email still.