Which of the following protocols is used by e-mail servers to send messages?

SNMP (Simple Network Management Protocol) is used for monitoring and managing network infrastructure devices such as routers and switches - it has no role in email message transmission.

FTP (File Transfer Protocol) transfers files between a client and a file server over TCP ports 20 and 21 - it is not used for sending email messages.

POP3 (Post Office Protocol version 3) is a retrieval protocol used by email clients to download messages from a mailbox on a server - it receives mail rather than sending it.

SMTP, defined in RFC 5321, operates over TCP port 25 for server-to-server relay and port 587 for client-to-server message submission, and it functions as a push protocol - the sending server initiates a connection to the receiving server and delivers the message. It handles all outbound email transmission, including routing through intermediate relay hosts. Every email sending path - from mail client to outbound server or from server to server - relies on SMTP.

HTTP (Hypertext Transfer Protocol) facilitates communication between web browsers and web servers for delivering web content - it is not the standard protocol for email server-to-server message delivery.