This sounds obvious but make sure port 80 is open on the firewalls and routers between sender and receiver.
It's easy to get bogged down in trying to work out why MSMQ isn't sending, such as checking if acknowledgements are getting lost and so on.
In this case take a step back and look at what you're trying to do - POST data over the HTTP protocol.
If the messages are stuck in the outgoing queue and nothing at all shows up in the web logs on the server then try a simple test - browse the destination machine with Internet Explorer to see if HTTP communication is possible.
Bottom line - If IE can't get through then MSMQ has ZERO chance of success.