How to verify if email exists or not using C#

I came across one github projects GitHub (https://github.com/aron-tn/bounce-checker)

It is supposed to be a software to clean email list and find bounce emails. It returns very accurate results for all emails domains, even the catch all domains like (yahoo, aol) and private domains. I was analyzing the php and python code (here is a screenshot: https://prnt.sc/zzl93m[^] ) and seems that the developer is just sending request to a web script (.ashx) that does all the verification and returns the result back in JSON.

This is the link for the web script (https://www.namecheap.com/Cart/ajax/Validation.ashx)

I was wondering what method he used to get such accurate results for all domains? Is there a public resource code for the same web app he is using? I want to study this for educational purposes.

What I have tried:

I reviewed pretty much all of the related questions and code resources available around that topics but I found nothing as accurate and as fast as the one made by this guy on github.