The phrase "mail back" can be replaced by a number of synonyms depending on the context. For example, "return via post," "send in the mail," "forward by post," and "post back" are all possible alternatives. In addition, "ship back," "return by courier," or "send by recorded delivery" can be used in situations where a more secure or faster method of delivery is needed. In some cases, "respond by mail" or "reply via post" might be appropriate for situations where an individual is being asked to provide information or documentation. Regardless of the context, there are many different synonyms that can be used to convey the basic concept of sending something back through the mail.