I think I remember a claim that asynchronous message passing can be implemented by synchronous message passing but not vice versa. Unfortunately, I don't remember an article name, an author, or even a field. Does this ring a bell, or am I delusional?
You can create a synchronous messaging system with an asynchronous messaging system. This is done by waiting for an acknowledgement directly after sending a message.
An example for this is the actor model which is able to build a synchronous messaging system like CSP.
But you also can simulate a asynchronous messaging system. When you use buffers to store the message. So the Receiver can access it when he wants to.
An example for this would be the email system which works with TCP.
In Synchronous, asynchronous, and causally ordered communications they make the statement that the simulation of both is a well known fact.
In Actors, a model of concurrent computation Agha makees the statement that synchronous messages are always build out of asynchonous messages.
EDIT: I changed my original answer, thanks to the comments. I really thought that Tanenbaum had made a comment on this, but he didn't and since i come from the actor model background i always assume message to be asynchonous even synchronous ones.