Could someone spell out a use case for this? One that comes to mind would be validating that a mailing list doesn't have any outdated email addresses in it, but couldn't that be determined by just checking for a bounce when a message is sent to it?

I tried to sign up for SiriusXM the other day, and though I could create an account with my .pro email address, I couldn't actually sign up for service with that same address for some reason. It's frustrating that validating email addresses is still something that people get so wrong. Please just take whatever seeming garbage I've entered into your email address field and try to send a message to it.

(Their site also had stupid password generation rules such that I couldn't use the 21-character one my password manager auto-generated, but even after I made one that followed the rules on the page, it was still rejected because there were apparently rules on the back end that weren't spelled out in the front end. Please hire me, SiriusXM.)

>but couldn't that be determined by just checking for a bounce when a message is sent to it

The fewer times you bounce, the better your chances of not being marked as a spammer.

This could also be useful for an ecommerce site, where you want to be able to easily contact the buyer if there's a delay, address correction needed, etc. People typo their own email at a rate that's surprising.

I use email validation via MailGun for exactly this purpose with a productized service business. If we don't have a good email, then we can't deliver the service once complete. That leads to angry customers, even if the issue was a typo when they created the account. Easier to try and catch it during signup rather than at the time of delivery when emails start bouncing and you have no way to get ahold of the person.

https://github.com/mailcheck/mailcheck

Fork modified for React: https://github.com/eligolding/react-mailcheck#readme