Exchange 2003 trough ISA

i have set up my exchange 2003 server to work through ISA 2004, i can send emails from the internal network to “outside” but i can not receive emails, tested it through yahoo and i got the following error:
213.175.172.178 does not like recipient.
Remote host said: 550 5.7.1 Unable to relay for j.mansour@shi.edu.lb
Giving up on 213.175.172.178.
Please Advise.