SMTP, or Simple Mail Transfer Protocol, is a standard communication protocol used for sending emails across the Internet. It defines how email messages are transmitted from one server to another, ensuring that they reach their intended recipients. SMTP operates on a client-server model, where the email client sends the message to the server, which then forwards it to the recipient's email server. SMTP works primarily on port 25, although other ports like 587 and 465 are also used for secure connections. It is essential for the functioning of email services, allowing users to send messages efficiently and reliably.