Useful to extend the SMTPAppender class in order to add some Email Headers such as X-Priority, In-Reply-To, ...

Example of usage:

... updateMimeMsg(...) { 

  Level highLevel = cb.asList().stream().map(ILoggingEvent::getLevel).max(comparingInt(Level::toInt)).orElse(Level.DEBUG);

   mimeMsg.setHeader("X-Priority", convertToPriorityString(highLevel));

}