Lotus Notes users are listed as Contacts in Exchange, so the Exchange users can address them easily and forwarding works.
1-Add the Lotus Notes user's email address as a proxyAddresses value to the contact in AD
2-The receive connector that Domino talks to (or Quest Coexistence Manager for Notes if you are using that) to be 'externally secured' - this will cause Exchange to look them up in the GAL and match them to the contact. Having the receive connector set to anonymous will cause this behavior.
Please see the attached document which outlines the correct receive connector settings.