Java mail overhead -- need help

Question

I am using javamail to send out email with attachments, but I found a heavy overhead using it. The file I am going to attached to mail is 13MB, but after I build MimeMessage object, it becomes 18MB. The following is piece of my code, any help is appreciated.
public static void send(String host, String to, String from, String subject, String message, Collection fileNames) throws GLPMException {
// create some properties and get the default Session
Properties props = System.getProperties();
props.put("mail.smtp.host", host);
Session session = Session.getDefaultInstance(props, null);
session.setDebug(false);
try {
// create a message
MimeMessage msg = new MimeMessage(session);
if (from != null) {
if (!from.trim().equals("")) {
msg.setFrom(new InternetAddress(from));
InternetAddress[] address = InternetAddress.parse(to, false);
msg.setRecipients(Message.RecipientType.TO, address);
if (subject != null) {
msg.setSubject(subject);
// create and fill the first message part
MimeBodyPart bodyPart = new MimeBodyPart();
if (message != null) {
bodyPart.setText(message);
Multipart multiPart = new MimeMultipart();
multiPart.addBodyPart(bodyPart);
// attach the files to the message
if (fileNames != null && !fileNames.isEmpty()) {
Iterator fileNameI = fileNames.iterator();
while (fileNameI.hasNext()) {
String fileName = (String) fileNameI.next();
MimeBodyPart bodyPart2 = new MimeBodyPart();
FileDataSource fds = new FileDataSource(fileName);
bodyPart2.setDataHandler(new DataHandler(fds));
bodyPart2.setFileName(fds.getName());
multiPart.addBodyPart(bodyPart2);
// add the Multipart to the message
msg.setContent(multiPart);
// set the Date: header
msg.setSentDate(new Date());
// check message size before send out, if it is greater than 15M
// populate warning message to user.
ByteArrayOutputStream bais = new ByteArrayOutputStream();
msg.writeTo(bais);
logger.log(Level.FINEST, "message's size " + bais.size());
if (bais.size() > 15 * 1024 * 1024) {
GLPMExceptionMessage exceptionMsg = new GLPMExceptionMessage(
GLPMExceptionType.WARNING, IErrorConstant.EMAIL_OVERSIZE_MSG);
throw new GLPMException(exceptionMsg, GLPMExceptionType.WARNING);
// send the message
Transport.send(msg);

Answer 1

Open the pdf file in notepad. If it contains the line drawing characters (������), foreign characters or little boxes, it is 'binary'. Text is considered to be characters 1-127. The line characters and foreigen characters and boxes are 128-255. PDF's are 'binary' in this sense. They will have to be encoded. A common standard is MIME64 encoding which you are using.
Read more here.
http://www.winnetmag.com/Articles/Index.cfm?ArticleID=4913