Register only via a specific e-mail domain I need to allow users to register for a Drupal-6 site, but only if their email contains a specific domain. For example, only allow users with whatever.com in their  email address to register; a user with the email bob@whatever.com can register for the site, but bob@something.com cannot register for the site.

Is there a module for this? If not, which file would the PHP need to be altered for this?
the "user restrictions module" is supposed to offer that feature. But it doesn't seem ready for production yet. However you could use the code to write your own.

I also tried to do it with the rules module and setup a rule for the "event" "user: before saving account" with a condition "data comparison" on field "account:mail". 
Add "value contains" and use "@whatever.com" and then check the "negate" box. That will basically trigger the rule for all emails that don't contain @whatever.com.

However I couldn't find a way to stop the username from getting saved. If you use the event "user: after saving account" you can show an error to the user and block the account. It prevents the registration but shows a second message to your user "you'll get notified by the administrator". Not a great experience for your customers.