public static function get_inbox_count( $user_id = 0 ) {
		global $wpdb;

		if ( empty( $user_id ) ) {
			$user_id = bp_loggedin_user_id();
		}

		$unread_count = wp_cache_get( $user_id, 'bp_messages_unread_count' );

		if ( false === $unread_count ) {
			$bp = buddypress();

			$unread_count = (int) $wpdb->get_var( $wpdb->prepare( "SELECT SUM(unread_count) FROM {$bp->messages->table_name_recipients} WHERE user_id = %d AND is_deleted = 0 AND sender_only = 0", $user_id ) );

			wp_cache_set( $user_id, $unread_count, 'bp_messages_unread_count' );
		}

		/**
		 * Filters a user's unread message count.
		 *
		 * @since 2.2.0
		 *
		 * @param int $unread_count Unread message count.
		 * @param int $user_id      ID of the user.
		 */
		return apply_filters( 'messages_thread_get_inbox_count', (int) $unread_count, $user_id );
	}