SYN and CWR ECE SYN

Hi,

I telnet a remote mail server smtp port 25 from internal lan, no problem.
The log in the Linux box has SYN packets destined to the remote mail server.
However, if I telnet that mail server from my Linux box, the packets to that
server is not SYN, it is CWR ECE SYN. And the remote mail server respond
with nothing back.
The linux box has 3 NICs, config as multiple uplinks with iproute, iptables
running.

SYN
PC- - - - - - - - -> --- NT4.0 --->linux------>remote mail server, telnet
OK.

linux- - - - - - - - - >remote mail server, telnet no good.
CWR ECE SYN

How do I fix this problem?
Any help will be appreciated.

Jason