Body is a TIdStrings property that represents the textual content of a MIME message part having one of the Text media types identified in the Internet Standards document: 

RFC 2046 - Multipurpose Internet Mail Extensions (MIME), Part Two: Media Types  

One of the text media types, like "text/plain" or "text/html", will be used in the ContentType property. TIdText is used when the ContentType property for a TIdMessage instance indicates a MIME-encoded message in the "multipart/*" media type family. 

For message that do not contain a ContentType in the "multipart/*" family, use the Body property in the TIdMessage instance to access the textual contents of the message. 

Body will be populated with the values from the TIdStrings instance passed an argument to the constructor. Body may also be maintained using inherited properties and methods from TIdStrings, like Text and Add.

Create 

TIdMessage.Body 

TIdMessagePart.ContentType