Which protocol is used to send emails?

Concept

Application-layer protocols in the TCP/IP model are each purpose-built for a specific kind of communication - one for transferring files, one for delivering hypertext, one for remote terminal access, and one for relaying electronic mail between mail servers. The protocol responsible for delivering (sending) email between mail servers is the Simple Mail Transfer Protocol (SMTP), defined in RFC 5321.

Application to this question

Here the question asks which protocol sends emails. SMTP supplies exactly this function: it defines the command sequence (HELO/EHLO, MAIL FROM, RCPT TO, DATA) that a sending mail server uses to hand a message to a receiving mail server, and it typically runs over TCP port 25 (or 587 for client submission).

Cross-check against the other protocols

• HTTP (Hypertext Transfer Protocol) governs how a browser requests and a web server returns hypertext resources such as web pages; it carries no mail-relay commands.

• FTP (File Transfer Protocol) governs uploading and downloading files between a client and a file server over separate control and data connections; it has no mail-header syntax.

• Telnet governs opening an interactive text-based session on a remote host, sending keystrokes and receiving character output over a virtual terminal connection; it carries no email envelope or message-relay semantics.

Since only SMTP defines mail-relay commands between mail servers, SMTP is the protocol used to send emails.