pramodkmr73
Go to Admin Panel > Manage > Forms, click the Contact Information Form, click Config next to Email Address, set the Validator to Regex, add /^[^@]+@gmail\.com$/ as the regex (updating gmail\.com with whatever their email domain is; yes, the \. is very important in regex), add a custom Validation Error (if needed), and click Save. This will only allow Users with the email domain of your choosing to register for an account, create tickets, etc.
Cheers.
