sshd not responding when accessing from outside of firewall

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Hi there,

I have two servers running redhat linux advanced server 2.1, both connected
to the same LAN. The two servers can ssh to each other without any problem.
However, when accessing from outside of firewall using ssh, timeout occurs.
The firewall has port 22 open and on my server side, I use tcpdump to
monitor the network traffic and find that there are packets from client
machine(located outside of firewall), but sshd is not sending any packet

Any guru here can give some enlightenment what is the reason for this?

Best regards,

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