I fundamentally disagree with you. If a piece of technology allows for such mistakes, then it is at fault, not people

It has to be accessible but the software limiting the user
"So far, we’ve been lucky. When technologies try to protect you from yourself too much, they tend to die out in a few years"

In order to avoid user mistakes, what should the email client do?

Email is powerful beyond any other medium precisely because it does not try to wrest control from its users.