Exchange 5.5 – error sending email

Running exchnage 5.5 behind MS-Proxy 2.0

When send to a particular address, it have the following error:

The following recipient(s) could not be reached:’sample_name@abc.on.ca’ on 7/4/2001 5:47 PM

Unable to deliver the message due to a communications failure

If we send it from hotmail, it does goes through, so the problem should be on our side.

Anyone?