Hi,
Which firewall we talking about ?
So you ping only the public IP address of Firewall. Because you are NATing there is no direct connection to the server ...
I think there are inboud filters on the firewall who block's this host (ip address) ....
Dubble check and compare with the other clients ....
If the ISP block ports, the only one I quess is port 25 (SMTP) ...
A/