Have you ever wanted to deny access to all users with a certain e-mail address? Or, better yet, have you ever wanted to only allow users with @institution.edu to create accounts on your site? Below an easy solution using Drupal's access rules.
View your access rules by going to Administer -> Access Control -> Access Rules.
Click on Add Rule and add the following e-mail rules:
- Deny %@% (which blocks all e-mail address)
- Allow %@institution.edu (which will allow people with firstname.lastname@example.org)
When you're done go and check your rules by clicking on "Check Rules." (Drupal 5)