Which of the following protocols is used to send e-mails on the Internet?

SMTP (Simple Mail Transfer Protocol) operates on TCP port 25 for server-to-server relay and port 587 for client submission, and is the only protocol in this list designed for sending and relaying email. IMAP4 and POP3 are retrieval protocols, and HTTP is a web transfer protocol, making SMTP the definitive answer for email transmission.

IMAP4 (Internet Message Access Protocol version 4) is used to remotely access, manage, and synchronize email stored on a mail server, not to send messages.

POP3 (Post Office Protocol version 3) is used to download email from a server to a local client for offline reading, not to transmit outgoing mail.

HTTP (Hypertext Transfer Protocol) is used for transferring web pages and web resources between browsers and servers, not for sending email.