The protocol used for sending email messages over the internet is called SMTP, which stands for Simple Mail Transfer Protocol. SMTP is a communication protocol that facilitates the transfer of email messages between servers and clients. It primarily handles the sending of emails, ensuring they are routed correctly to recipients based on their email addresses. While SMTP handles message transmission, email retrieval is typically managed by protocols like IMAP or POP3. SMTP is fundamental to the functioning of email services in modern communication.