How to encrypt and sign emails using Hide My Email addresses
Apple introduced the Hide My Email feature with macOS Ventura in late 2022. Hide My Email is built in to Sign in with Apple and iCloud+.
GPG Mail 7.1 introduced support for OpenPGP encryption and signing of emails using the Hide My Email service. By default Apple disables S/MIME encryption when users switch to a Hide My Email address.
In order to use OpenPGP with your Hide My Email address, please create a new key in GPG Keychain using that exact email address. When composing a new draft, GPG Mail will look for a key matching the address and when a key exists for one of your Hide My Email addresses, show the encrypt and sign buttons.
When no key exists matching the email address the buttons will not be visible.
GPG Mail FAQ
GPG Keychain FAQ
GPG Services FAQ