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 SMTP, which stands for Simple Mail Transfer Protocol. SMTP is specifically designed for the transmission of email over the Internet, facilitating the process of sending emails from the sender's email client to the recipient's email server. It operates by establishing a connection between mail servers and using a series of commands to relay messages.

When an email is sent, the client utilizes SMTP to communicate with the email server. The server then processes the email and might store it for later retrieval by the recipient, or it might forward it to the recipient’s server. This is why SMTP is pivotal in the email-sending process; it ensures that messages are transferred accurately and efficiently across the network.

In contrast, other protocols listed serve different functions. HTTP (Hypertext Transfer Protocol) is used for transferring web pages and other resources on the World Wide Web. FTP (File Transfer Protocol) is designed for transferring files between computers on a network. IMAP (Internet Message Access Protocol), on the other hand, is utilized for retrieving emails from a server rather than sending them. Each of these protocols has its distinct purpose, making SMTP the correct choice for sending emails specifically.