[drnick1]

Email has never been immutable. Email that you receive on your own server can be trivially altered, it's just a plain text file.

In fact until recently email was sent and received in the clear like a postcard, the whole system wasn't designed to be secure or secret in any way.

[lexlambda]

Immutable as in the message won't be altered/deleted by the sender. This is. This is about user control, as opposed to chat apps or social media, where posts are frequently edited, get taken down after an outrage or links can disappear to link-rot.

From article: "An email is your copy, and the sender can’t revise it later."