Postfix send mail for specific domain through vpn
I have a working postfix mail server.
I bougt a cisco VPN router which has established vpn connection to remote mail server.
Now i want mail for domain example.com that users behind my linux server send through postfix to be sent through established VPN tunnel.
So my linux has internal ip of 192.168.20.20 and outside ip x.x.x.x
Cisco is visible from linux on internal ip 192.168.21.20 and vpn from it is to the remote mail server of ip adress y.y.y.y.
So do i have to add to transport map for postfix (/etc/postfix/transport) line like:
? Would that be correct or do i need to do something more ?
Thanks for the help.