Which of the following ensures that a sender of a message is who they claim to be?

Authentication is the process that ensures the sender of a message is indeed who they claim to be. This is crucial in information systems and communication, as it verifies the identity of users or systems attempting to access services or information. In the context of messaging, it involves the use of credentials, such as passwords or digital certificates, which are checked against a database to confirm that the identity presented is legitimate.

When a sender successfully authenticates, recipients can have a level of confidence that the message originated from the stated sender, thereby reducing the risk of impersonation or fraud. This measure is fundamental in establishing trust in communications, particularly in platforms like Microsoft Exchange Server, where secure and reliable communication is paramount.

The other concepts, while related to security in communication, do not specifically address the verification of identity. Authorization deals with granting access rights to authenticated users, encryption focuses on protecting the privacy of the message content, and nonrepudiation ensures that a sender cannot deny having sent a message once the message has been delivered and received.