iptables NAT entry times out but connects from firewall

[Hanasaki JiJi <hanasaki@hanaden.com>]

There is a firewall with two NICs and the below rule to allow an
internal host to connect out to smtp servers on the internet.  Some
hosts have a connection timeout on a connect from $INTERNAL_IP_OF_SMTP
yet connect from the firewall just fine.

iptables -t nat -A POSTROUTING -p tcp -o $NIC_EXTERNAL \
       --dport 25 -s $INTERNAL_IP_OF_SMTP -j MASQUERADE

ex:
on firewall:
	telnet csoc-mail-msfc.csoconline.com 25
	
	above connects ok

on $INTERNAL_IP_OF_SMTP
	telnet csoc-mail-msfc.csoconline.com 25

	connection times out