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The Simple Mail Transfer Protocol (SMTP) is the standard protocol used for sending email messages from a mail client, such as Outlook or Thunderbird, to a mail server. SMTP is specifically designed for transmitting email over the Internet and operates on a client-server model. When a user composes an email and hits send, the mail client communicates with the mail server using SMTP to deliver the message to the recipient's email server.

Other protocols listed have different functions: IMAP (Internet Message Access Protocol) and POP3 (Post Office Protocol version 3) are both used for retrieving emails from a mail server to a mail client but do not deal with sending messages. IMAP allows multiple devices to access and manage emails in sync, while POP3 usually downloads emails for offline storage and then deletes them from the server. HTTP (Hypertext Transfer Protocol) is used primarily for transferring web pages and other content on the World Wide Web, not for email transmission.

Thus, SMTP is the correct choice as it directly handles the sending of email messages from the client to the server.