Shade one lozenge to indicate the application layer protocol used for sending emails from a client device to a mail server.

The TCP/IP model consists of several layers, and the application layer contains protocols for specific tasks. Let's look at the options: - **FTP (File Transfer Protocol):** This protocol is used for the bulk transfer of computer files between a client and server on a network. - **HTTP (Hypertext Transfer Protocol):** This is the foundation of data communication for the World Wide Web, used to load web pages. - **SMTP (Simple Mail Transfer Protocol):** This is the standard internet protocol for sending electronic mail (email) from a client to a server, and between servers. - **UDP (User Datagram Protocol):** This is a transport layer protocol, not an application layer one. It provides a connectionless and unreliable service, suitable for applications like video streaming or online gaming. The question specifically asks for the protocol used for **sending emails**, which is the definition of SMTP.