Do you want to only allow people with certain email address to create account on your website?
e.g if you want to only allow people with the email address @gmail.com to create accounts on your Drupal website.
Here is the way to do that using Drupal’s access rules.
Login to your Drupal site as admin
Go to Administer=>Access Control=>Access Rules
Click on tab ‘Add Rule’ and add the following email rules:
Deny %@% (This will block all email address)
Now we want to allow users only with email address @gmail.com