Index: src/wp-includes/user.php
===================================================================
--- src/wp-includes/user.php	(revision 49064)
+++ src/wp-includes/user.php	(working copy)
@@ -1453,9 +1453,20 @@
 function email_exists( $email ) {
 	$user = get_user_by( 'email', $email );
 	if ( $user ) {
-		return $user->ID;
+		$user_id = $user->ID;
+	} else {
+		$user_id = false;
 	}
-	return false;
+
+	/**
+	 * Filters whether the given email exists or not.
+	 *
+	 * @since 5.6.0
+	 *
+	 * @param int|false $user_id The user's ID on success, and false on failure.
+	 * @param string    $email   Email.
+	 */
+	return apply_filters( 'email_exists', $user_id, $email );
 }
 
 /**