Can receive messages, but can not send out

My biztalk can receive messages from partner's server , but can not send 
messages out.

When the partner (a webMethods server) sends me a 2A1 message, my server 
receives it and saves it in the BTARNDATA database MessagesToLOB table.
But 
the partner does not receive my Ack message. There's an error in my event
log:

A message sent to adapter "HTTP" on send ****t "x.Async" with URI 
"http://localhost/BTARNApp/RNIFSend.aspx?TPUrl=https%%3a%%2f%%2fb2b-ecommerce-test.x.com%%2finvoke%%2fwm.ip.rn%%2freceive&xRNResponseType=async"

is suspended. 
Error details: The remote server returned an error: (400) Bad Request. 
MessageId: {A186FBEB-16CE-4644-8813-CF82017CCA00} 
InstanceID: {5C5F663E-7A84-4730-BED1-1567E441449D} 

When I try to send a 3A4 message to the partner, the same error occurs.


Any idea?
Is it a configuration problem on my side or the partner's side?
Is there any tool to find out the problem?

Regards,
wuz