At what point in a work-email exchange (non-work friend) is it ok to stop using introductions (e.g. "Dear You-" or "You-") and closings ("Thanks, Me" or "Sincerely, Me")?

If someone responds to a comment or post with a question that starts with "um", that person is:

Do you like posting your response to inane polls that show just how open-minded and enlightened you are?