The delta chat application should have a feature which will enable users to initiate a self disruptive messages. A user can decide to send a message that disrupts after his chosen time frame. It can also be set to disrupt after being seen
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not sure if this is possible with email, if there where a way to have a key that expires after one use, maybe delta chat could generate a temporally private key that isn’t available to the user and that if destroyed after the message is decrypted… 
That should be set in the server side. Regartheless was implemented in the client side to delete them on each user server, someone with keys (one of the users) from the server side or who created a modified client could avoid disrupt and save messages.
Protonmail solved this by not sending a real email, but a link with the content
So I assume this can’t be done with any email.