Re: The way things are going with Office 36x (where x >>> 0)
"Not true for at least a decade. Exchange is specifically built to support multiple tenants. You can easily run it as a shared hosted service with multiple tenants that are utterly unaware of each other."
That's a very different thing to what @SchorThing was talking about. In Thing's scenario, the 35 different companies want to be aware of each other, and share a common directory for that purpose. They have different top-level certificates (/O), but there will be cross-certificates between them.
In Exchange/AD you'd need a ridiculous complexity of trust relationships between the different domains in the forest to achieve the same thing.
What Domino administrators can happily do on a Friday afternoon, knowing AdminP is looking after stuff ------->>>>