In the above scenario of Exchange Server, the administrator should use S/MIME.
What is S mime tells?
S/MIME is a short form of the word Secure/Multipurpose internet Mail Extensions. This is known to be a famous and well used protocol that is often employed in transferring or sending digitally signed and encrypted messages.
Note that the S/MIME is often used in Exchange Online as it helps to give a lot of services for email messages such as Encryption.
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