What ItemPropertyPath is needed to get the sender of an mail via EWS?

Question

We want to get the sender of an mail address via C# and EWS. We use Independentsoft´s Exchange Web Service for this task. It works fine by the following code:

var lItem = m_Service.GetMessage(ItemId);
return lItem.Sender.Name;

The Problem: When requesting the sender of thousands of messages, this could last a long time. It is much faster when we only request the necessary item properties:

var lItemPropertyPaths = new List<PropertyPath>() { ItemPropertyPath.? }; // -> replace the '?' by any property    
var lItem = m_Service.GetMessage(ItemId, lItemPropertyPaths );
return lItem.Sender.Name;

The question is: What do we have to insert for the '?' in ItemPropertyPath.? to get the sender?