email_whitelist

This module helps you to specify a list of email address (comma separated values). After you save the settings, During user-registration process, this module will compare the email address entered with the list saved by admin. If it matches, only then the new user can successfully register. Example: efgh@gmail.com,"abcd@example.com",ijkl@xyz.com (so, only these email addresses are allowed to register in the site)

Apart from email address, you can also specify host names (comma separated values) in the similar way.
Example: yahoo.com,gmail.com,example.com,adf.com,asdf.in (so, only email addresses with these host names are allowed to register)

There is also a "negate" option - which does the opposite. If this option is enabled, it allows any email/host other than the listed ones.

There is "Access Rules" for the similar kind of functionality, but this is much more user friendly and works directly with csv files.