"EOM is pretty superfluous, though -- it's clear from the fact that there's no more text that I've reached the end of the message."

Originally the idea was to put EOM at the end of the subject line when the whole message fit in the subject. That way you didn't have to 'open' the email. Never made much sense to me, since the process of 'opening' an email doesn't cost more than the effort to select the subject, but maybe it depends on the email client - I can see how one on a 800x600 screen may use an email client that shows the message list in one screen, and the body in another.