CRM 2011 : Deliver promote Plugin event

Hi,

I am just updating last contacted info on contact record if inbound email contains a contact in to,cc or bcc.

For that I have registered plug-in on email DeliverPromote Pre-Event. I am using below code 

if (context.PrimaryEntityName == "email")
                {
                    if (context.PrimaryEntityId != null)
                    {
                        Entity email = service.Retrieve("email", context.PrimaryEntityId, new ColumnSet("to", "cc", "directioncode"));

                        if (!email.GetAttributeValue<bool>("directioncode"))
                        {
                            if (email.Contains("to"))
                            {
                                traversecollection(ref ContactIDs, email.GetAttributeValue<EntityCollection>("to"));
                            }

                            if (email.Contains("cc"))
                            {
                                traversecollection(ref ContactIDs, email.GetAttributeValue<EntityCollection>("cc"));
                            }
                        }
                    }
                }

traversecollection is just a function which extracts a ID if Entity in entity collection is of type contact. But while I track email from outlook to CRM. it just throws error "Invalid argument". 

Am I doing anything wrong ?

Thanks and regards,
Hiren Solanki.
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