One important clarification since the question comes up often. AMP for email doesn't make emails inherently mutable.

Emails already support updating by including external image references inside the email. Allowing this is a choice of the client.

AMP for email makes JSON requests proxied by the email client. Your client can decide to only make these requests once and then keep them stable or to show you a version history of the state of your email upon every time you opened it.