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S/MIME (Secure/Multipurpose Internet Mail Extensions) is a standard for digitally signing and encrypting email messages using public-key cryptography and X.509 certificates. When you receive an email with a digital signature icon or a padlock in Outlook, Gmail, or Apple Mail, that's S/MIME at work — proving the sender's identity and optionally encrypting the message so only the intended recipient can read it.
The standard builds on PKCS #7 / CMS (Cryptographic Message Syntax) to wrap email content in signed or encrypted envelopes. A signed message includes the sender's certificate and a cryptographic signature over the message body — recipients can verify the signature using the sender's public key. An encrypted message is encrypted with the recipient's public key from their certificate — only their private key can decrypt it. Messages can be both signed and encrypted.
S/MIME is the enterprise alternative to PGP/GPG for email security. It uses the same X.509 certificate infrastructure as HTTPS, which makes it natural for organisations that already have a PKI (Public Key Infrastructure). Certificate management is handled by IT departments, email clients support it natively (Outlook, Apple Mail, Thunderbird), and many compliance frameworks (HIPAA, financial regulations) accept S/MIME as a valid email encryption mechanism.