During Exchange setup, one of the questions you have to answer is whether there are any pre-Outlook 2007 clients in your environment. (I wrote briefly about this before in the context of Office Communicator.) However, do you know what happens when you click “yes” or “no”?
If you click “yes”, the setup program will create a public folder database, in which you’ll find the familiar Schedule+ Free/Busy and Offline Address Book folders. This shouldn’t be a surprise; Outlook 2003 and earlier versions require these folders, so you’d expect Exchange to create them. If you click “no”, the public folder database isn’t created, so pre-Outlook 2007 versions can’t get free/busy data or download the OAB. However, what I didn’t know until today is that the Exchange store will also block MAPI connections from older versions of Outlook when you say “no”. Why? Because if those clients did connect, they’d have a terrible experience, with no free/busy or OAB support. To reduce the support hassle for themselves and Exchange 2007 admins, MS decided just to block the connection. To fix this, just add a public folder store to your server and voila! you’re golden.