Relaying question

[Joseph Mays]

We have long had a postfix system for a cluster of machines that accepts 
incoming mail from a spam filtering system, and sends outgoing mail directly 
out to other servers in the world. I am currently trying to change it to 
send outgoing mail out through the smtp filter server, too. However, if I 
change the relayhosts on the cluster postfix config to the ip number of the 
mail server, any thing sending outgoing mail through the cluster machines 
gets a response of "relaying denied" from the cluster machines. If I take 
the relayhosts line out so that the cluster machines just directly send mail 
to the outgoing world, again, the problem goes away.