I was trying to view the SOAP messages and headers that were actually exchanged with a certain webservice.
To do so, I used Fiddler2... available from http://fiddler2.com/.

Fiddler2 will allow you to monitor traffic when using HTTP, SOAP or WCF ports in BizTalk. However, Fiddler will not trace any messages sent to endpoints by BizTalk by default as it does not use WinInet. To overcome this issue, you'll need to configure a proxy to allow Fiddler to intercept the messages.
Note that these proxy settings will need to be removed when Fiddler is not running. Because otherwise all traffic through this proxy will not be received by anything.

In the send port of BizTalk go to the Configuration settings. Open the tab 'Proxy' to configure the necessary settings. Now you should change following settings:
  • Server: 127.0.0.1
  • Port: 8888

That's is all you need to do. Now open fiddler2 and process a message. If all is configured correctly, you should see the exchanged messages coming through fiddler.