Simple Mail Transfer Protocol (SMTP) is a standard communication protocol used for sending and receiving email over the internet. It defines how email messages are transmitted between servers and how they are delivered to the recipient's inbox. SMTP operates on a client-server model, where the email client sends messages to the server, which then forwards them to the recipient's email server. SMTP works primarily on the Transmission Control Protocol (TCP) and typically uses port 25 for communication. It ensures that emails are sent reliably and efficiently, allowing users to communicate seamlessly across different email services and platforms.