Restricting domain of email address as login name

We are setting up our internal/intranet Plone site for our company. We would like to use self-registration and have users use their email addresses as their login name. However, we would like to restrict the allowable email addresses to be our company provided email addresses.

I can't find any setting to do that and neither reading documentation nor Google Fu has yielded any results. Does anyone have any suggestions?

We use a monkey patch for doing a custom email validation in one project:

from import BaseRegistrationForm

def validate_registration(self, action, data):

    # pick email from from data and validate against CiviCRM
    result = user_by_email(data['email'])
    if not result:
        err_str = _(u'Email not found in CRM')
        notifyWidgetActionExecutionError(action, 'email', err_str)
    # perform original validation
    return old_validate_registration(self, action, data)

# monkey-patch registration validation
old_validate_registration = BaseRegistrationForm.validate_registration
BaseRegistrationForm.validate_registration = validate_registration