XF 2 Only Allow Certain Email/Domain to Register

Hello All!
I'm wanting to build a forum for a group of people who all have a common email domain (example: tim@example.com, john@example.com, cathy@example.com, etc). What would it take and cost to develop an addon that will restrict user to only use @example.com email addresses for user registration. If they do not have that domain in their email I do not want them to be able to register. And if they fail to provide that email I want them to get an error stating they must register with their @example.com email address.

Please let me know if I need to provide more information in regards to this for clarity!
Thank you!
 
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