I use Telnet for verifying the email address.
See screenshot of how I verify the email address using Telnet.
The problem is that after verifying some thousand email addresses. The IP's gets listed in the blacklists. Once they are in blacklist no further email verification happens.
I have tried to run the verification through multiple IP ranges of around 16 IP's but almost every IP got listed in blacklist after verification of few thousand emails.
I want to get the email verification done in huge numbers without getting in blacklist.