Index: wp-admin/user-new.php
--- wp-admin/user-new.php
+++ wp-admin/user-new.php
@@ -41,7 +41,7 @@
 	check_admin_referer( 'add-user', '_wpnonce_add-user' );
 
 	$user_details = null;
-	if ( false !== strpos($_REQUEST[ 'email' ], '@') ) {
+	if ( is_email( $_REQUEST[ 'email' ] ) ) {
 		$user_details = get_user_by('email', $_REQUEST[ 'email' ]);
 	} else {
 		if ( is_super_admin() ) {
Index: wp-includes/user.php
--- wp-includes/user.php
+++ wp-includes/user.php
@@ -469,7 +469,7 @@
 			if ( $qv['search_columns'] )
 				$search_columns = array_intersect( $qv['search_columns'], array( 'ID', 'user_login', 'user_email', 'user_url', 'user_nicename' ) );
 			if ( ! $search_columns ) {
-				if ( false !== strpos( $search, '@') )
+				if ( is_email( $search ) )
 					$search_columns = array('user_email');
 				elseif ( is_numeric($search) )
 					$search_columns = array('user_login', 'ID');
Index: wp-login.php
--- wp-login.php
+++ wp-login.php
@@ -187,7 +187,7 @@
 
 	if ( empty( $_POST['user_login'] ) ) {
 		$errors->add('empty_username', __('<strong>ERROR</strong>: Enter a username or e-mail address.'));
-	} else if ( strpos( $_POST['user_login'], '@' ) ) {
+	} else if ( is_email( $_POST['user_login'] ) ) {
 		$user_data = get_user_by( 'email', trim( $_POST['user_login'] ) );
 		if ( empty( $user_data ) )
 			$errors->add('invalid_email', __('<strong>ERROR</strong>: There is no user registered with that email address.'));