When you are running an Exchange server, and have your users access the server from outside the company network, you usually have to open up access to OWA. This enables users to access their mail, contacts and appointments by using an internet browser. Occassionally, someone also has a ActiveSync enabled phone that they would like to use. This protocol also uses OWA for syncing data. And very rarely, there's also a guy (usually the CEO) who simply wants to use its Outlook to connect - this is possible through web as well using RPC-over-HTTP(S), but that's information for another blog post (on   ...