S/MIME applies cryptographic protection to MIME message content. It can provide digital signatures, encryption, or both.
A signature can provide integrity and identify the certificate used to sign. Encryption protects content for intended recipients who possess the corresponding private keys.
Operational reality
The cryptography is only part of deployment. Organizations also need certificate issuance, trust, renewal, private-key protection, recovery, directory/discovery strategy, and client compatibility.
Losing an encryption private key can mean losing access to historical encrypted content unless recovery was designed in advance.