Obviously the email was sent encrypted to other account and you don't know
how to decrypt it. The question is rather if you really wish to encrypt
emails that you send?
As if you wish, you need to generate secret and public keys for both email
accounts of yours. The receiving account must have the secret key to decrypt
the email that was sent to him.
You always use public keys of the receiver to send emails to them.
Receivers always use secret keys to decrypt and read the emails.
There is excellent manual for you to understand they basics:
https://gnupg.org/gph/en/manual.html
