Assembles the body of the message, Attachments are attached as mime multipart objects with Content-Disposition: attachment;
Also determine whether this message falls within the constraints specified, such as sizing,
@param message the message to be used in the assembly
@param attachments the attachments for the message
@return a new MimeMultipart message containing allmessage bodies.
@throws MessageException thrown if there are any problems retrieving theassets.