Which protocol ensures that email can be sent securely?

The choice of SMTPS as the correct answer relates to its role in ensuring the secure transmission of email. SMTPS, or Simple Mail Transfer Protocol Secure, is an extension of the standard SMTP protocol. It uses Transport Layer Security (TLS) to provide encryption for emails while they are being sent from the email client to the email server and between servers. This encryption is crucial for maintaining the confidentiality and integrity of the messages being transmitted, thus preventing unauthorized access or interception by malicious actors.

In contrast, SMTP itself does not include any built-in security features for encrypting transmissions, making it vulnerable to eavesdropping. Alternatives like POP3 and IMAP primarily focus on retrieving email from a server rather than sending it securely. While they also have secure versions (POP3S and IMAPS, using TLS), they do not address the initial sending of an email, which SMTPS specifically protects. Therefore, SMTPS stands out as the protocol designed explicitly for secure email transmission.