TIdMessagePart = class(TIdCollectionItem);

TIdMessagePart is a TCollectionItem descendant that encapsulates a MIME message part. TIdMessagePart is the ancestor class for common MIME message parts like TIdText and TIdAttachment. TIdMessagePart is also the collection item added to the TIdMessageParts collection. 

TIdMessagePart provides methods and properties common to all MIME message parts, including properties that represent values from Internet Message headers and the content for the message part. 

Instances of TIdMessagePart are not normally created during normal usage. Use one the descendant classes, like TIdText or TIdAttachment.