HELP: Postfix under firewall ...

Hello.

I have first postfix-mail on the gate (domain sibir.su) and second
postfix into intranet (sk2.sibir.su - 192.168.0.100)
On the gate-postfix:
main.cf:
transport_maps = hash:/usr/local/etc/postfix/transport
relay_domains = $mydestination sk2.sibir.su

transport:
sk2.sibir.su relay:[192.168.0.100]

send mail to user@sk2.sibir.su from internet receive message:
"test@sk2.sibir.su Unrouteable address"
What is wrong?

Best regards,
Sergey.