There are many modules for allowing users to register and login by email address. One is Email registration
But, what about when you want to allow or restrict the email registration by specific domain? For example, to only allow/deny registration from users with @example.com email addresses.
An easy workaround without depending on any contributed module is by using "Access Rules".
Add the following e-mail rules:
- Deny %@% (which blocks all e-mail addressess)
- Allow %@yourdomain.com
Sometimes tasks which seem to be difficult are very easy!