Bob, your senior colleague, has sent you a mail regarding a deal with one of the clients. You are requested to accept the offer and you oblige. After 2 days. Bob denies that he had ever sent a mail. W

Authentication verifies the identity of a user or system at login time, but it does not provide a persistent, verifiable proof that a specific message was sent after the fact.

Confidentiality ensures that message content is kept secret from unauthorized parties, but it says nothing about proving the identity of who sent the message.

Integrity ensures that a message has not been altered in transit, but it does not prove the identity of the original sender or prevent the sender from denying authorship.

Non-repudiation provides proof of the origin of a message so that the sender cannot later deny having sent it. In email systems, this is implemented using digital signatures - where the sender signs the message with their private key - allowing any recipient to verify authorship with the corresponding public key and present that proof irrefutably. This is the only security property that directly addresses the scenario of proving who sent a specific communication.