[Logician]

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.