What is the purpose of STARTTLS in SMTP?

• A. STARTTLS upgrades an existing plaintext connection to TLS.
• B. STARTTLS is used to sign emails.
• C. STARTTLS encrypts at rest.
• D. STARTTLS is a replacement for SMTP.

Answer: (A)

STARTTLS in SMTP is about upgrading an existing plain-text connection to a secure TLS channel. When an SMTP session begins, the client and server talk over a plain connection. If the server supports STARTTLS, it advertises that capability, and the client can request to switch to TLS. When the server agrees, a TLS handshake happens over the same connection, turning the ongoing session into an encrypted one. From then on, SMTP commands and mail content are protected in transit.

This is not about signing emails (that’s done with digital signatures like S/MIME or DKIM), not about encrypting data stored on servers (encryption at rest), and it’s not a replacement for SMTP itself—the mail protocol remains SMTP, but the transport is secured with TLS after the upgrade.