First of all, Before purchasing, please test live demo preview and we are always here to provide support within support limitation. Follow our troubleshooting documentation to optimize your VPS!!
On the other hand, few of the main reasons why it might not be working for your server are,
You might be using a shared hosting or reseller service with service limitations from host. Best pick would be a personalized VPS instead of shared hosting. Because in shared / reseller hosting there are too many restrictions get applied from WebHost side but not all of them have problems!
A cheap vps hosting will do the work. You can use free panel like centoswebpanel, tinycp, cyberpanel etc.
Also, email verification or checker only use default localhost mail server and SMTP port 25 should be open. Email that you assign as your account email address get used to send an HELO response to target email addresses. So make sure that email address is valid and exists. If your localhost mail server being relayed from external mail server and not under same IP address then you might get this issue also. So make sure your localhost mail server SMTP using the default IP of existing hosting server. This supposed to be a default setup in a VPS, no configurations needed for it.
Alternatively, you can try any different web hosting server, which does not use SMTP relay email through an external gateway.
As script does not come with free customization works, So we do not provide any customization works of our script for this issue, as it’s a default process of how our script should works now. But we have a plan in future to add multi smtp support, where you may add your custom settings for email scan and sending email. Otherwise, you may contact us for a paid service to modify the script, especially for your hosting environment.