Simple? How mail via smtp to seperate Domino domain using Notes address from secondary address book?

This should be simple … but it’s got me foxed!

We use R6. The partner (on a different domain) uses R5. We hold a copy of their address book on our mail server configured as a secondary nab through directory assistance. We mail externally only through a SMTP relay. Problem is that the server sees the address in directory assistance and insists on routing it locally – and won’t send it out. Internet mail routes out fine. Unknown addresses route out fine. Can I configure addresses of the partner domain to route out via smtp relay?