Protonmail (and I guess all others including Gmail, except Fastmail) has a nasty feature, where the sender can put an expiration date on emails and practically get a confirmation you received the email without you ever knowing you received the email.

If the expiration is for example one day, you might never see it.

To my knowledge, Protonmail does not even show information that the email has expiration. Nor can you access log of deletions.

This feature was used against me on a court trial.

I am working on an email spam filter - would it be interesting to you? An extension perhaps?