Lotus Domino Messaging & Collaboration Express licenses should allow a single, two-server cluster for DR purposes. Even if it req's an 'upgraded license' of some type.
Domino Express licenses are a very nifty way to get & deploy N/D for smaller customers.
BUT if you want to do something very simple, like have a backup server at a DR site that's 'hot' and ready, you enter a pretty complicated licensing situation.
Domino clustering is (honestly) ...