Index: wp-includes/formatting.php
===================================================================
--- wp-includes/formatting.php	(revision 17950)
+++ wp-includes/formatting.php	(working copy)
@@ -1512,7 +1512,7 @@
 
 	// LOCAL PART
 	// Test for invalid characters
-	if ( !preg_match( '/^[a-zA-Z0-9!#$%&\'*+\/=?^_`{|}~\.-]+$/', $local ) ) {
+	if ( ! preg_match( '/^[a-z0-9!#$%&\'*+\/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&\'*+\/=?^_`{|}~-]+)*$/i', $local ) ) {
 		return apply_filters( 'is_email', false, $email, 'local_invalid_chars' );
 	}