This class represents a part of the message. Each part has
MessageHeader and body.
As structure of a MIME message is recursive, body can in turn consist of one or more parts.
There are 7 top-level types according to the standard: text, image, audio, video, application, multipart and message. Use TElMultiPartList when you deal with multipart type. TElPlainTextPart is used for parts of "plain" subtype of the "text" type. But its main porpose is compatibility with old non-mime messages.