Final answer:
The statement about S/MIME is true as it uses MIME protocol encoding and adds digital signatures based on public-key cryptosystems to secure emails, providing authentication, message integrity, and non-repudiation.
Step-by-step explanation:
The statement is true - Secure Multipurpose Internet Mail Extensions (S/MIME) does indeed build on the encoding format of the MIME protocol and incorporates the use of digital signatures to ensure the security of e-mail communications. S/MIME takes advantage of public-key cryptosystems to provide end-to-end security services for electronic messaging.
This includes authentication of the sender's identity, message integrity (ensuring that the content hasn't been altered during transit), and non-repudiation (providing proof that the sender actually sent the message).