SMTP is the principal application-layer protocol for Internet electronic mail. It uses the reliable data transfer service of TCPto transfer mail from the sender’s mail server to the recipient’s mail server. As with most application-layer protocols, SMTPhas two sides: a client side, which executes on the sender’s mail server, and a server side, which executes on the recipient’s mail server. Both the client and server sides of SMTP run on every mail server. When a mail server sends mail to other mail servers, it acts as an SMTPclient. When a mail server receives mail from other mail servers, it acts as an SMTPserver.