code: protected void populate( MimeMessage message )
      throws MessagingException
    {
      // add all of the to addresses.
      message.addRecipients(Message.RecipientType.TO, toSet.toInternetAddressArray());
      message.addRecipients(Message.RecipientType.CC, ccSet.toInternetAddressArray());
      message.addRecipients(Message.RecipientType.BCC, bccSet.toInternetAddressArray());
      message.setFrom(from);
      if( replyTo != null ) {
        message.setReplyTo(new InternetAddress[] {replyTo});
      }
      if( subject != null ) {
        message.setSubject(subject);
      }
    }
docs: Populates a mime message with the recipient addresses, from address, reply to address, and the subject.