Causes of HTTP 400 Bad Request error

I'm trying to send an AS2 XML message via request/response on the HTTP
adapter. For now, I have a test sender application set up on my
BizTalk server, and the receive application I've developed. Set up
the application pool to run under an identity that has admin rights,
added it to the IIS_WPG group, copied the BTSHttpReceive.dll to the
web site dir and enabled it as an ISAPI extension, I think I've done
everything to set it up right.

But when my sending application tries to send, I get a response 400
Bad Request. What things should I be looking at to diagnose this?
It's driving me crazy, any help would be appreciated.