I know someone who always changes the subject line to reflect the content. He even changes the subject line when responding to other people's e-mails, which is

I know someone who always changes the subject line to reflect the content. He even changes the subject line when responding to other people's e-mails, which is quite annoying because it often breaks Gmail conversations. So you'll have to take some measure to tell people not to delete that passphrase from the subject line. For example, "Don't change or delete this or it will go into my spam box." But that's verbose.