private Guid GetAndClearSalesLiteratureID(ICrmService service, Guid EmailID)
{
    TargetRetrieveEmail target = new TargetRetrieveEmail();
    target.EntityId = EmailID;
    RetrieveRequest getemail = new RetrieveRequest();
    getemail.ReturnDynamicEntities = true;
    getemail.Target = target;
    getemail.ColumnSet = new AllColumns();
    RetrieveResponse retrieved = (RetrieveResponse)service.Execute(getemail);
    DynamicEntity dynEntity = (DynamicEntity)retrieved.BusinessEntity;
    if (!dynEntity.Properties.Contains("new_salesliteratureid"))
    {   // failed???
        return Guid.Empty;
    }
    Microsoft.Crm.Sdk.Lookup id = (Microsoft.Crm.Sdk.Lookup)dynEntity.Properties["new_salesliteratureid"];
    // and clear...
    id.IsNull = true;
    id.IsNullSpecified = true;
    service.Update(dynEntity);
    // done
    return id.Value;
}