Which protocol is used to send email messages from a client to a server?

The protocol used to send email messages from a client to a server is Simple Mail Transfer Protocol, commonly known as SMTP. This protocol facilitates the process of transferring mail between the client's email application and the email server, enabling users to send their emails effectively. SMTP operates primarily on TCP port 25, ensuring reliable message delivery over the internet.

While the other protocols mentioned serve different purposes, they do not handle the sending of emails in the same way that SMTP does. For instance, File Transfer Protocol (FTP) is designed for transferring files between hosts, Hypertext Transfer Protocol (HTTP) is used primarily for transferring web pages, and Post Office Protocol (POP) is intended for retrieving emails from the server to the client's email application, rather than sending them. Therefore, SMTP is the definitive protocol for sending emails.