what is smtp? option simple mail transfer protocol (smtp) is the protocol responsible for sending e-mail messages from an e-mail client to an e-mail server. option simple mail transfer protocol (smtp) is the protocol responsible for enabling files to be downloaded to a computer or uploaded to other computers. option simple mail transfer protocol (smtp) is the protocol responsible for preparing data for transmission and providing for error checking and resending lost data. option simple mail transfer protocol (smtp) is the specification to simplify attachments to e-mail messages