Index: src/wp-includes/user.php
===================================================================
--- src/wp-includes/user.php	(revision 49123)
+++ src/wp-includes/user.php	(working copy)
@@ -1585,8 +1585,9 @@
  * Conditional Tags} article in the Theme Developer Handbook.
  *
  * @since 2.0.0
+ * @since 4.9.0 Introduced `username_exists` filter.
  *
- * @param string $username Username.
+ * @param string $username The username to check for existence.
  * @return int|false The user's ID on success, and false on failure.
  */
 function username_exists( $username ) {
@@ -1598,11 +1599,12 @@
 	}
 
 	/**
-	 * Filters whether the given username exists or not.
+	 * Filters whether the given username exists.
 	 *
 	 * @since 4.9.0
 	 *
-	 * @param int|false $user_id  The user's ID on success, and false on failure.
+	 * @param int|false $user_id  The user's ID associated to the username, 
+	 * or false if the username isn't associated to a user.
 	 * @param string    $username Username to check.
 	 */
 	return apply_filters( 'username_exists', $user_id, $username );
@@ -1623,9 +1625,20 @@
 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 );
 }
 
 /**