edit register.php, find:
PHP Code:
if (!$allowregistration) {
before that add:
PHP Code:
if (stristr($email, '@')!='@yourdomain.com')
{
eval("standarderror(\"".gettemplate("email_notallowed")."\");"); exit;
}
(edit yourdomain.com)
AdminCP/Add New Template, template name: email_notallowed
Create your message which will be displayed to users who doesnt have this email according to your wish.