Well, I'd actually have to say that you are jumping the gun by assuming they didn't read your email. From your post it sounds like you're working in Outlook. If that's the case, it's quite likely that the recipient could have read the message without outlook ever having registered it as having been read.
If they have their email options set with a message preview pane, as soon as your emil is the one that is 'highlighted' in their inbox, the text of the message appears in the preview pane where they can read it there. Once they click off of that email and onto another one (or if the click to reply to or fwd your email) it will register that email as read. However, if they don't click onto another message before deleting your message, it will never register that they read your email and you'll get a tracking reciept stating that your message was deleted without having been read, even though they read it when it was being displayed in the preview pane.
I don't think you were out of line to email that person, since it's an important email they needed to read; however, you shouldn't go to HR or take any offense without more proof that the email was never read.