The Simple Mail Transfer Protocol (SMTP) is the network protocol commonly used to send email messages. SMTP facilitates the transfer of messages from a sender’s email client to the recipient’s email server, ensuring reliable delivery. This protocol primarily handles the sending process, while other protocols, such as IMAP and POP3, are used for retrieving and managing email messages. SMTP's standardized framework ensures seamless communication between different email systems, playing a critical role in the functionality of the email ecosystem.