消息系统：保存并发送 - 何时执行新消息的插入？

In a private messaging system built with PHP and MySQL users can compose new messages and answer to messages. There are two buttons: "SEND" and "SAVE". The message status is stored in the database (0=not yet sent, 1=sent). Also, the message is being saved automatically every e.g. 3 minutes via an ajax-call.In a private messaging system built with PHP an