AttachmentEncoding is a String property that identifies the encoding mechanism used for message parts that are attachments in the message. 

AttachmentEncoding normally contains values that include: 

Changing the value in AttachmentEncoding updates the default encoding used for all message parts in the message by setting the same value in the MessageParts collection. This also ensures that a registered encoder class type is located for subsequent handling of the content in the message parts. 

Set the value in Encoding to indicate the encoding mechanism for the message Body.

TIdMessageParts.AttachmentEncoding 

TIdMessage.Encoding