Re: Why everyone uses envelopes but few encrypt emails?

Except that the MITM now has impersonated you and has taken your secret
message as well. Knowing this after the fact is way too late.

The fact that the recipient can find out that the message has been
spied upon is a deterance for spies. This is similar to an envelope -
you only know that someone has opened the envelope on your letter after
the fact.