For example, I would like to add the In-Reply-To field myself, because I want to respond to an archived message.

  • It would be helpful to know what macOS version you have and if you have a preference as to which mail client you would like to do this with. Flexibility on that latter would be good as I don't believe mail.app allows editing headers. – Steve Chambers Mar 4 at 14:28
  • @SteveChambers I only have Mail.app and Microsoft Outlook installed. I have heard that Thunderbird can do this, but would prefer not to switch at the time. Mail.app can show headers for incoming mails, so I thought it makes sense to allow adding headers for outgoing ones. – Franklin Yu Mar 4 at 15:04

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