Websites don't have an actual check for a legit email... That's why they send confirmation links to click. Forms check that it ends with @something.something, sometimes they can add filters to not allow anything other than the 20 most popular domains. Or they can block addresses with less than 3 characters, because nobody real has those. But most forms you can keyboard smash anything@anything.anything. They just might make a temp account until it's confirmed or something.
But joeblow@aol.com almost certainly was/is real.
Edit: looks like I was also incorrect!
Wow thank you for the info, I didn't know they had such advanced lookups.