Use of XPC Services from mailKit

Thanks for this excellent session about Mailkit at WWDC. Using the sample code from the session, I added a new XPC Service Target and am trying to call remote objects via the NSXPCInterface from within mailComposeSessionDidBegin. No success. I can verify that the XPC Service is working fine when I make the same calls to the service from the AppDelegate class' applicationDidFinishLaunching function. So, my question is if mailKit extensions are blocking XPC service invocations?