从msdn上看二者的解释：
postmessage ： Places (posts) a message in the message queue associated with the thread that created the specified window and returns without waiting for the thread to process the message.
To post a message in the message queue associated with a thread, use the PostThreadMessage function.
There is a limit of 10,000 posted messages per message queue. This limit should be sufficiently large. If your application exceeds the limit, it should be redesigned to avoid consuming so many system resources. To adjust this limit, modify the following registry key.
就是异步函数，但是投递数量有限制。
sendmessage:
Sends the specified message to a window or windows. The SendMessage function calls the window procedure for the specified window and does not return until the window procedure has processed the message.
To send a message and return immediately, use the SendMessageCallback or SendNotifyMessage function. To post a message to a thread's message queue and return immediately, use the PostMessage or PostThreadMessage function.
可以看出sendmessage是同步的。