I've mentioned XSO (Exchange Server Objects) here a couple of times, wondering what was happening to this managed API for Exchange. Well, according to a post by Tim Anderson, it looks like the future is web services as far as accessing Exchange is concerned. To be expected, I suppose. Tim mentions the prospect of being able to write Java and Compact Framework clients, but he also mentions that Exchange already has WebDAV support and WebDAV can be used cross-platform. The problem with WebDAV is that it is a fairly complex protocol which needs an API (not provided by Microsoft) whereas with a web service I presume you'll be able to load the WSDL into your IDE and generate the code to access the service.