Mail delivery wierdness

[keith at smmc.qld.edu.au]

Hi all,
I am trying to connect to an exchange server via my new gateway ipfilter
fbsd 4.7 ipnat firewall.
Telnetting to the server inside to port 25 reveals...

Connected to 203.44.yyy.xx
Escape character is ']'.
Connection closed by foreign host.

Why would it close instantly? Does this reveal while I can't send mail to
any email account on it? Is this an exchange hassle? Is port 25 to be only
tcp or udp as well.
Thanks
keith