Does anyone know how to mark an email being sent by JAVA as private?

It seems that if there was a way of doing this it would be done using a message flag, eg:

javax.mail.Message m = new MimeMessage(sess);
m.setFlag(Flags.Flag.DELETED, true);

However, the flag constants offered are DELETED, ANSWERED, DRAFT, FLAGGED, RECENT, SEEN and USER.

Can anyone help?