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

The protocol used to send email messages from a mail client to a mail server is Simple Mail Transfer Protocol (SMTP). This protocol is specifically designed for the transmission of email across networks. When a user sends an email, the mail client utilizes SMTP to transfer the message to the respective mail server, which may then relay it to another server or deliver it to the intended recipient.

In contrast, Internet Message Access Protocol (IMAP) and Post Office Protocol version 3 (POP3) are primarily used for retrieving emails from the server, rather than sending them. IMAP allows users to access and manage their email on the server, while POP3 downloads emails to the client's local device and typically deletes them from the server. The File Transfer Protocol (FTP) is a standard network protocol used for transferring files between a client and a server, but it is not used for email transmission. Thus, SMTP is the correct choice when discussing the protocol responsible for sending emails from a client to a mail server.