Exchange 5.5 multiple mail return

I have Exchange 5.5 setup on a NT4.0 server running thru a PIX515 firewall & T1 connection. When my clients attempt to send mail to multiple addresses they get the host unreachable error back. I can sit at the Exchange queue and send them each on by one without error before they time out. I had thought it was a firewall problem, but worked with CISCO and do nt believe that to be true now. Please advise….thanks