Just in case the user included the wrong public key, or doesn't know how to decrypt the email they received, the plaintext one should give them instructions on what to do.
Or, possibly, this could be an extra MIME part in the encrypted message itself that isn't encrypted. Then it can all be sent in one email, however it's unclear how email clients will deal with this.
Just in case the user included the wrong public key, or doesn't know how to decrypt the email they received, the plaintext one should give them instructions on what to do.