Limit user registration by email domain
You can use domain_registration to allow or dissalow emails from certain domains to register.  

Setup is very simple as seen from the instructions on the project page


  
  Install the module 
  visit 'admin/config/system/domain_register'
  Choose if you want to only allow, or disallow the domains from registering 
  Enter the domain names you wish to allow/deny (one per line) in the format of 'foo.com'. Wildcards are allowed. 
  Enter the error message you wish to show to the end user if the email address
  doesn't validate