I created this plugin for a couple of client projects and thought it might be useful for others.
The User Domain Whitelist plugin limits user registration to only registrants with an email address from the domain white list provided by the administrator. For example, firstname.lastname@example.org would only be allowed to register if example.com appeared in the domain white list. Anyone attempting to register using an email address outside the white list will receive an error message. Both the domain whitelist and the error message can be modified via the plugin options page (available under the Settings menu).