You don’t need Exchange to route Domino mail via SMTP. You can set up Domino server to route mail via SMTP.
The following link (below high-level instructions) contains more detailed instructions:
1. Prepare your system for sending messages to the Internet by testing your Internet connection and verifying that DNS is set up properly.
2. Enable SMTP routing within the local Internet domain so that servers can send mail over SMTP within the local Internet domain.
3. Enable SMTP to be used to send messages outside the local Internet domain.
4. Specify the SMTP relay host to be used to direct emails. If needed, configure a relay host for SMTP servers that do not have direct access to the Internet.
5. Set up inbound and outbound mail restrictions to protect against misuse of the mail infrastructure.
Remember, test, test, test.