S/MIME (Secure/Multipurpose Internet Mail Extensions) is a standard for end-to-end encryption and digital signing of emails. Unlike PGP, S/MIME relies on X.509 certificates issued by a certificate authority, which simplifies trust validation in corporate environments. It protects the confidentiality and integrity of business communication. In an ISMS, email encryption falls under the controls for information transfer (ISO 27001 Annex A 5.14). Ensure you have a working certificate-management process so that expired certificates do not disrupt email traffic.