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When you create a to-do in Mail using a selection of an email message, the URL for this message is added to the notes section of the corresponding item in Things. Clicking on it will open the associated email message in Mail.
— Things.app Blog
When you create a to-do in Mail using a selection of an email message, the URL for this message is added to the notes section of the corresponding item in Things. Clicking on it will open the associated email message in Mail.— Things.app Blog