ExtraHeaders is a TIdHeaderList property that represents message headers not used for with property storage. ExtraHeaders do not have corresponding properties to represent their values in the object instance. 

ExtraHeaders is used to store header values for the message that are not represented as properties of TIdMessage. ExtraHeaders is generally used to store values that might be added by a Mail User Agent, or for custom extension tokens beginning with "x-". 

Use the Methods and Properties in TIdHeaderList to maintain the custom message headers. 

When a message part is sent using a protocol transport provider, like TIdSMTP or TIdNNTP, values in ExtraHeaders are merged with values in the Header property during transmission. The same action is performed when calling the SaveToFile or SaveToStream methods.

TIdHeaderList 

TIdMessage