The Workflow Foundations are more suitable for... well.. work flows representing the business logic. You seem to be more interested in integrating existing services. In that case I would focus more on the Windows Communication Foundation. Which were introduced together with the (original) Workflow Foundation. So WCF is part of .NET Versions 3 and above. WCF will support Webservices, Message Queueing and FTP out of the box. For email you will probably need some kind of custom channel. However, it's highly likely that someone else has already written one for you.
According to Microsoft:
"Windows Communication Foundation (WCF) is a framework for building
service-oriented applications. Using WCF, you can send data as
asynchronous messages from one service endpoint to another. A service
endpoint can be part of a continuously available service hosted by
IIS, or it can be a service hosted in an application. An endpoint can
be a client of a service that requests data from a service endpoint.
The messages can be as simple as a single character or word sent as
XML, or as complex as a stream of binary data."
This page on MSDN contains resources to help developers get up to speed on developing with Windows Communication Foundation (WCF). Keep in mind, all of this concerns only the communications layer. You'll have to write glue-code yourself.
If you really want a more "framworky" solution, I'd agree with Tommy Grovnes. 'Service Bus' is the buzzword to search for. In fact Stackoverflow already provides many insights on what services busses are good for, what implementations exist, experiences with them and so on. Especially this question on .NET service busses looks like a promissing start.
Cheers
dave