You can still use all of those features in Outlook, but you just can't share them across the network using a Unix MTA. We don't even know if he needs collaboration. Plus, like I said earlier, Domino does everything Exchange does, and you can still use Outlook as the client.
I actually know Exchange better than I know any of the Unix MTA's or Domino, but I don't think its worth the money. Exchange plus Windows servers are very expensive and aren't worth it at all IMO. If I need collaboration, I would pick Domino in a second. Read this article from Lotus.com...
"iNotes Access for Microsoft Outlook
iNotes Access for Microsoft Outlook allows Microsoft Outlook users to access their mail, calendar, and scheduling information using a Domino server instead of Exchange. With iNotes Access for Outlook, organizations can take advantage of the Domino capabilities, such as reliability, scalability, and expanded platform coverage for their messaging infrastructure without changing clients. Additionally, as of 5.0.5, iNotes Access for Microsoft Outlook utilizes the Domino Off-Line Services (DOLS) technology to not only help in the delivery of the client code to the end users, but also to enable end users to access their mail while disconnected."
Who said Helfenmir even *bought* Outlook anyway? Most people who have Outlook, didn't purchase an Outlook license, they purchased a M$ Office license and Outlook came included with it. And of course, there is the other way of getting it (which is my preferred way of getting M$ products)...
ChrisP