发送消息后，客户端可保留和修改，而不会影响到已发送邮件。相同的消息对象可以多次发送。
在它的发送方法的执行，决不能由客户端进行修改的消息。如果被修改，发送的结果是不确定的。
After sending a message, a client may retain and modify it without affecting the message that has been sent. The same message object may be sent multiple times. During the execution of its sending method, the message must not be changed by the client. If it is modified, the result of the send is undefined.