Qmail error

VLAN and Firewall implementation were successful, there is no problem with sending and receiving email local and internet mail. Everything were running smoothly until one day, all clients from VLAN cannot send mail to internet, they can only send tolocal email, but they can receive internet mail. I already added all the vLAN gw to /etc/rc.local. What other configuration should I check or modify? Here is the message from the client “553 sorry, that domain isnt in my list of allowed RCPTHOSTS”

Hope someone could help me