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Those blue lines?
These are an indication of quoted text. If you have received something that was pasted in as a quote in some earlier forwarding of the message OR if the message originator copied something from some source and then pasted it into the email message as a quote OR the original source is simply forwarded, it will appear as quoted text, hence the vertical line alongside. This is not unique to Outlook. It's a feature of many, if not all, email programs.