The protocol used for sending emails is known as Simple Mail Transfer Protocol (SMTP). SMTP is a communication protocol that facilitates the transfer of email messages between servers. When a user sends an email, SMTP defines how the message is packaged and transmitted to the recipient's server. Additionally, SMTP operates alongside other protocols like IMAP (Internet Message Access Protocol) and POP3 (Post Office Protocol), which are used for retrieving and managing emails on the recipient's side. Together, these protocols ensure effective email communication across different systems.