What is SMTP protocol? Explain.

SMTP (Simple Mail Transfer Protocol) is the standard protocol for sending emails across the Internet. SMTP is an application layer protocol which is used by mail servers to send, receive, and relay emails between senders and receivers.

The SMTP standard defines the conversation between the sender of the e-mail and the SMTP mail server that delivers the mail. In a SMTP conversation, the sender issues a certain sequence of commands to the receiving SMTP server. Generally, if the sender issues valid commands and the intended recepients of the e-mail are valid accounts on the receiving server, the receiving server will accept the message and attempt to deliver it.