I would use this to determine how likely a given email address will receive my mail. I build websites, where people create accounts and an awful lot of them provide unroutable email addresses. This is annoying for me and for the users. If I could give them a warning like "this email address looks like it isnt usable, are you sure?".
Yes the coverage is not going to be near 100% so you’d get a lot of anger with this use case. There are some libs that will check via smtp if they are likely reachable, and some services. Is “verify your email” and option ie sending an email on signup as part of the signup flow an option?
I’m interested in what you think you’d use it for if you don’t mind sharing, and what you would benchmark it against.