What is SMTP$?$

Simple mail transfer protocol $($SMTP$)$ is the protocol responsible for sending e$-$mail messages from an e$-$mail client to an e$-$mail server.

Simple mail transfer protocol $($SMTP$)$ is the protocol responsible for enabling files to be downloaded to a computer or uploaded to other computers.

Simple mail transfer protocol $($SMTP$)$ is the protocol responsible for preparing data for transmission and providing for error checking and resending lost data.

Simple mail transfer protocol $($SMTP$)$ is the specification to simplify attachments to e$-$mail messages.