A MimeMultipart is obtained from a MimePart whose primary type is "multipart" (by invoking the part's getContent() method) or it can be created by a client as part of creating a new MimeMessage
non puoi fare un MimeMultipart con new e passargli una stringa
devi arrivarci dal messaggio
body = parts.getBodyPart(i);
MimeBodyPart mimePart = (MimeBodyPart)body;