Hi Dan,
As Riquel mentioned, since the application you want to design is coupled with Exchange server, it would be helpful to get more information on the Exchange development side(some existing SOA or REST service API exchange has already opened).
For WCF side, it just provides a unified programming platform for you to build SOA conformed application/services. If you want to get more about WCF service design or patterns, you can have a look at the MSDN WCF developer center or the service station series articlesin MSDN magazine:
#Windows Communication Foundation
http://msdn.microsoft.com/en-us/netframework/aa663324.aspx
#MSDN Magazine: Service Station
http://msdn.microsoft.com/en-us/magazine/cc501058.aspx
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