Make sure you have applied Rollup 1, Server, client and router:
http://www.microsoft.com/downloads/details.aspx?FamilyID=57c6267b-3b13-49dd-bfed-3cc83633aea7&displaylang=en
Read http://support.microsoft.com/kb/956129
Symptom 3
If it still does not work try below:
The problem has been documented to occour when the users account does not have enough privillages. So ensure that the network security settings allow the router to create the email activity on the CRM Server.
Hassan.