Stuck with creating a SaaS App

Hi Coders,
I am a full-stack developer, still learning new stuff. I am now trying to build my own saas app but what I am seeking from you is to help me with some information.

I am trying build an webapp like this Minelead, Hunter. These are mainly providing email verification service for helping companies to collect leads. But I have gone through about how their service worked and I found some info like checking MX records, sending VRFY command, need to 25 port open etc. And I found some code in github and stackoverflow but most of them are not working anymore. And one my team mate write an script but every time he send request in SMTP server, he get blocked.

from dns import resolver
from smtplib import SMTP as smtp

email = ""
domain = ""
mx_record = resolver.query(domain, 'MX')
mail_exchangers = [exchange.to_text().split() for exchange in mx_record]
dest_pair = (mail_exchangers[1][1], 25)
sm = smtp(dest_pair[0])
sm.helo()
sm.mail(email)
smtp.rcpt(email)

And I searched over the youtube for any single hints about it but I am failed. One of my friends give me an idea and said

Most email severs will now allow you to do this at all without sending an email. It’s considered a security vulnerability and is actually the way Trump"s Twitter was hacked. The hackers need to figure out the email he used so they brute forced the SMTP Verification. That’s why it is almost always disabled or limited

But if so then how other saas app does that? And they are not 2 or 3, they are hundreds of site out there. Just searching free email verify

I lost so many weeks after this and totally disappointed. Any thoughts?

Thanks

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