Depending on where you live, you could claim the ad was binding, but now you have no email to refer to. You could probably make a screenshot, but OP has a point

There's a reasonable expectation that a website may get updated. The same is not true for mail.

Emails already support updating by including external image references inside the email.

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.

> Emails already support updating by including external image references inside the email.

I think this is not comparable. If an email links to an external image, the external image is not the email, it's a link and behaves as everyone expects a link to behave. The email itself is immutable.

This may seem to you like hair-splitting, but I don't think it is, since blocking external images is a common thing for people to do anyway. Doing so doesn't make email useless.

I realize you're arguing in the opposite direction, but removing mutable web history is one of the biggest justifications behind the push for IPFS and DAT.