A signed message is multipart/signed with an application/pkcs7-signature part (a detached CMS SignedData) or an opaque application/pkcs7-mime blob; encrypted mail is CMS EnvelopedData or, in S/MIME 4.0, AuthEnvelopedData with AES-GCM. Senders discover recipients' certificates from earlier signed mail or directories, which is the main usability hurdle.
Publicly trusted S/MIME certificates have followed the CA/Browser Forum S/MIME Baseline Requirements since 1 September 2023, with mailbox-, organization-, sponsor- and individual-validated types and legacy, multipurpose and strict profiles (policy OIDs 2.23.140.1.5.x.y); new certificates may no longer use the legacy profiles since July 2025. The certificate must contain the address in subjectAltName and the emailProtection EKU.
S/MIME and OpenPGP solve the same problem with different trust models; S/MIME dominates in enterprises and governments that already run PKI. Post-quantum S/MIME is being defined through ML-KEM in CMS (RFC 9936) and ML-DSA certificates (RFC 9881). The S/MIME verifier checks a signed message locally.