If these are virtual machines (uml, xen, vmware, whatever) then the host could do IP NAT so that traffic _looks_ like it's coming from the host (or another of the virtual machines). If they're physically seperate boxes then you need to arrange for a smart-host type setup and have mail forwarding through that.
Nope, just one server with some domains. One server with the dns, sendmail and domains all bundled up in a nice pile. I am gonna try to do the different ip listen thing with sendmail since they all have their own ip first...
Gonna sleep now... A long night of getting no where with the sendmail books...well at least the issue is solved finally.. Whew.