Test server can't send mail outside domain

[nilocpay]

Hi,

I am running a test Exchange 2k server on my home network behind a
firewall.The Win2k domain is not registered, and I managed to send and
receive mails between users in the domain.However I could send a mail
out to any mail account(yahoo, hotmail etc) outside my domain.A test
the other way round would definitely not work, as the domain is
private and the ports for email were not enabled anyway.Below is the
"Undeliverable" message received :-

Your message did not reach some or all of the intended recipients.

Subject: test
Sent: 6/2/2002 4:18 AM

The following recipient(s) could not be reached:

nilocpay@yahoo.com on 6/2/2002 4:18 AM
The e-mail system was unable to deliver the message, but did not
report a specific reason. Check the address and try again. If it
still fails, contact your system administrator.

Other details which might be useful -

1)Nslookup able to resolve yahoo.com
2)telnet to yahoo port 25 seems ok
3)the box has 1 NIC only

Any advice would be appreciated.Thanks.

Regards
nilocpay