TheCthulhu

https://haveibeenpwned.com/ is a good site for checking if your email has been breached across the myriad of data dumps over the years.

This is an 80% of the way there good solution for validating email addresses. There's the chance people deleted their email addresses. I've come to assume most sales/marketing automation companies have already added this to their database as well as all other databases of hacked emails that have been publicly published.