Hi!
If you allow your host (MTA) to act as an SMTP relay, you need to configure the relay options as follows:
# Allow local and remote network to relay through this machine (at least it working for me)
Code:
mynetworks = 192.168.10.0/24, 127.0.0.0/8
Just add or make correct changes to above line and restart postfix.
ALSO note that conifg is diffrent for virtual domains.. are you config virtual domains??