How can I use Devise Authentication to validate that a sign up email address is from certain domain?

I would like to make sure the only people with email address of a certain domain can signup for a site that is using Devise.

For instance if people sign-up with the email joe@mysite.com, they should get a confirmation email but if the sign up with joe@yoursite.com, they should get a error message.

Uncommenting this line in config/initializers/devise.rb

# Regex to use to validate the email address
# config.email_regexp = /^([\w\.%\+\-]+)@([\w\-]+\.)+([\w]{2,})$/i

and changing it to use the domain I wanted to limit to:

config.email_regexp = /\A([\w\.%\+\-]+)@mysite\.com\z/i

did the trick.