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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 '19 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 '19 at 15:04
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UserHeaders

For Apple's Mail.app, you can use the default UserHeaders to provide a dictionary of key value pairs to insert into the headers of outgoing e-mails.

To set the Disposition-Notification-To header to email@address the following command can be used:

 defaults write ~/Library/Containers/com.apple.mail/Data/Library/Preferences/com.apple.mail.plist UserHeaders '{"Disposition-Notification-To" = "email@address"; }'

On macOS 10.14, you will need to grant Terminal.app the Full Disk Access right in System Preferences.app > Privacy & Security before issuing the above command.

See Mail Receipt for an example of this default and notes about its application.

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