property AttachmentEncoding: string;

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.