Explanation
S/MIME stands for Secure/Multipurpose Internet Mail Extensions, which is a standard for encrypting and digitally signing email messages. S/MIME can provide confidentiality, integrity, authentication and non-repudiation for email communications. S/MIME can encrypt the emails at rest, which means that the email contents are protected even if they are stored on the mail servers or the user inboxes. S/MIME can prevent email contents from being released should another breach occur, as the attacker would not be able to decrypt or read the encrypted emails without the proper keys or certificates. Verified References:
* Cryptography Concepts - SY0-601 CompTIA Security+ : 2.8
https://www.professormesser.com/security-plus/sy0-601/sy0-601-video/cryptography-concepts-2/ (See S/MIME)
* Mail Encryption - CompTIA Security+ All-in-One Exam Guide (Exam SY0-301)
https://www.oreilly.com/library/view/comptia-security-all-in-one/9780071771474/sec5_chap14.html (See S/MIME)
* Symmetric and Asymmetric Encryption - CompTIA Security+ SY0-501 - 6.1
https://www.professormesser.com/security-plus/sy0-501/symmetric-and-asymmetric-encryption/ (See S/MIME)