ltipart class is an implementation of the abstract Multipart
 class that uses MIME conventions for the multipart data. <p>

 A MimeMultipart is obtained from a MimePart whose primary type
 is "multipart" (by invoking the part's <code>getContent()</code> method)
 or it can be created by a client as part of creating a new MimeMessage. <p>

 The default multipart subtype is "mixed".  The other multipart
 subtypes, such as "alternative", "related", and so on, can be
 implemented as subclasses of MimeMultipart w