I'm not sure you're going to find too much support for that use case.
PGP encryption/signing of mail is primarily used for transmitting information between two people, and allowing the recipient to guarantee the sender that claims to have sent, sent it, that what was sent is what was received (signing), and, if necessary it was protected from prying eyes (encryption). As such, encryption/signing/etc is typically done in the specific user's mail agent, rather than at the SMTP server.
You would need whatever creates the automated email to sign/encrypt before sending it to postfix for further transmission.
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