The protocol used for sending emails over the internet is Simple Mail Transfer Protocol (SMTP). SMTP is an application layer protocol that facilitates the transmission of emails from a sender's mail server to the recipient's mail server. It defines the standards for how emails are sent, including the communication between different servers. SMTP is commonly used alongside other protocols like IMAP or POP3 for retrieving messages from mail servers, ensuring a complete email delivery system that supports various functionalities, including attachments.