Set FQDN to Exch on Cluster

I have a an Exchange 5.5 server running on an NT 4.0 cluster. Some anti SPAM filters are rejecting emails because the exchange server is not presenting an FQDN. I have spent a few hours going thourgh the IMC settings and searching the Net without luck. Can someone tell me where I set the FQDN for an Exchange server running on a Cluster? And if it is currently not set, what are the ramifications of setting one?

Thanks,

Pat