code: public String getMessageInfoType(BaseMessage externalMessage)
      {
          String strSubject = (String)externalMessage.getMessageHeader().get(EmailMessageTransport.SUBJECT_PARAM);
          if (strSubject != null)
              if (strSubject.toUpperCase().startsWith("RE:"))
                  return MessageInfoTypeModel.REPLY;   // This is (most likely) a reply to my message.
          return super.getMessageInfoType(externalMessage);
      }
docs: Is this message a reply (or a request).
  @param externalMessage The incomming message.
  @return MessageType.MESSAGE_IN or MessageType.REPLY_IN If this is a reply.