Which protocol is used to send email over the Internet?

Internet email delivery is split across two protocol families: push protocols that transfer a message from a sender's device or mail server onward to the recipient's mail server, and access/retrieval protocols that let a client pull mail down from a mailbox. SMTP (Simple Mail Transfer Protocol) is the standard push protocol used for outgoing mail; POP3 and IMAP are the retrieval protocols used on the receiving side. Protocols outside this family, such as file-transfer or web-page-delivery protocols, are not designed to carry mail at all.

For the protocol used to send email over the Internet, the transfer from a mail client to its outgoing mail server, and from that server onward to the recipient's mail server, is carried out over SMTP. That single-protocol match is why SMTP is the correct choice here.

• FTP moves arbitrary files between two hosts; it has no concept of a mailbox, message header, or mail-server hand-off, so it cannot deliver mail.

• HTTP fetches and serves web content such as pages and APIs; a browser-based webmail interface may use HTTP or HTTPS to talk to the webmail application, but that only covers the browser-to-webserver leg, which is a separate transport hop and not the protocol carrying the mail itself.

• The option claiming more than one protocol applies overstates it: among FTP, SMTP, and HTTP only SMTP fits the mail-delivery role.

• The option claiming none of the protocols apply understates it: SMTP does fit, so that option is wrong too.

This is exactly the point behind a common doubt about whether HTTP is used when sending mail over a browser: a webmail client, such as a browser-based inbox, does use HTTP or HTTPS between the browser and the webmail server, but once that message leaves the webmail application, the webmail server hands it off to the destination mail server using SMTP; the HTTP leg never reaches across the Internet to the recipient's mail server. So the Internet-wide protocol that actually sends the email is still SMTP, confirming the saved answer.