Email courier is an individual who gets out of their chair and approaches someone's desk or workstation in a work environment almost immediately after sending them an email, usually to make sure that the email has been received and that they can get a reply right away.
Bill: I just sent you an email. Did you get it?
Mike: Probably, I haven't checked.
Bill: Can you check?
Mike: Uh yeah, looks like I got it.
Mike: My immediate thoughts are you're an email courier and a douchebag.