can't receive responses

[David]

I have Windows Messenger 5.0, I can send messages all day
and the recipients receive my messages. The recipients
responses are bounced back as undeliverable.

I have blackice installed but disabled it, but no change.
It only happens when I am connected to the network
wirelessly, over ethernet it works fine

Both the recipients and I are using exchange, since this
is at work we have this problem. I realize that makes it
harder to determine the problem, but I thought this was
worth a try