The mail server has a specific definition of an unknown host. It considers a host unknown only if it looks up the DNS host name and receives an authoritative "Host does not exist."
Notice that a "Host does not exist" reply is not the same as receiving no reply on a query. Receiving no reply is considered a failed lookup and does not tell the server that the host definitely does not exist. It may be the case that the host actually does exist, but the lookup failed as a result of troubles with the DNS server, or because of network problems. For this reason, the mail server will hold the mail and attempt to look up the name again later.
Since the server must receive a response from a DNS server to consider a host unknown, this means that running the mail server without any DNS server will result in no mail being forwarded. One way to work around this is to run MacDNS for the mail server. For example, if your mail server is receiving mail for "company.com" and you want all such mail forwarded to another server, you can create a MacDNS zone file for "company.com" and create no hosts for that zone. Since MacDNS considers itself the authority for company.com, it will give a "Host does not exist" reply for any query about a host in the company.com domain. Keep in mind that this will also prevent the mail server from contacting any host in the company.com domain that actually does exist.