Index: wp-includes/ms-functions.php
===================================================================
--- wp-includes/ms-functions.php	(revision 18843)
+++ wp-includes/ms-functions.php	(working copy)
@@ -504,7 +504,7 @@ function wpmu_validate_user_signup($user_name, $user_email) {
 	$limited_email_domains = get_site_option( 'limited_email_domains' );
 	if ( is_array( $limited_email_domains ) && empty( $limited_email_domains ) == false ) {
 		$emaildomain = substr( $user_email, 1 + strpos( $user_email, '@' ) );
-		if ( in_array( $emaildomain, $limited_email_domains ) == false )
+		if ( ! apply_filters( 'limited_email_domains_check', in_array( $emaildomain, $limited_email_domains ), $emaildomain, $limited_email_domains )  )
 			$errors->add('user_email', __('Sorry, that email address is not allowed!'));
 	}