MSMQ 3.0 redirect problem

I'm testing MSMQ 3.0 via HTTP. Our network configuration is as follows:
A. Windows 2003 Server Edition in Site1
Queue messages are sent from here.

B. Windows 2003 Web Edition in Site2
This server has a public IP;
In this PC is installed an ISAPI filter called ISAPIRewriter that
forwards all HTTP messages to C.

C. Windows 2003 Server edition in Site2
This PC is configured with a public queue and HTTP queue support.

My tests have outlined three situations:

1. A sends a generic HTTP request to C (through B)
In this case, B receives the message, forwards it to C which replies
with a "200 OK" and the response arrives correctly to A

2. A sends an MSMQ 3.0 HTTP request to C (through B)
In this case, B receives the message and replies with "400 Bad request"
without forwarding the request to C

3. B sends an MSMQ 3.0 HTTP request to C
In this case the message arrives to C wich replies with "200 OK"

Now, my question is why in case 2 I get the "400 Bad request" error? Could
it depend on the fact that the message is discarded by IIS on B?
Is there a way to forward MSMQ messages from B to C without installing a
public queue on B (I can't install a public queue on B anyway, bacause it's a
Windows 2003 Web Edition machine)?

Thanks in advance for your support.