Via https://wordpress.org/support/topic/accept-also-smime-keys/page/2/#post-9174319: > It should be possible to upload the private key of the blogs admin e-mail so that e-mails can be sign when being sent out. Possibly, see [`openssl_pkcs7_sign()`](https://php.net/openssl_pkcs7_sign).
Via https://wordpress.org/support/topic/accept-also-smime-keys/page/2/#post-9174319:
Possibly, see
openssl_pkcs7_sign().