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> checkin to my flight button

You can do that in HTML email too. Newsletter have an "unsubscribe" button, calendar invites have a yes/no/maybe button.

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Not without leaving the email UI, no.

Let me introduce you to Actions and Highlights, which do exactly this without leaving the email UI. AFAIK Gmail has the best (only?) support for them, but anyone could support them.

https://developers.google.com/gmail/markup/actions/actions-o...

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