SMTP stands for “Simple Mail Transfer Protocol.” It is a connection-oriented, text-based network protocol from the internet protocol family and is located on the seventh layer of the OSI model: the application layer. Just like any other network protocol, it contains rules for correct communication between computers in a network. SMTP is responsible for feeding and forwarding e-mails from sender to recipient.
Since its release in 1982 as the successor of the “Mail Box Protocol” in Arpanet, SMTP has become the standard protocol for sending e-mails. For the average consumer, however, the SMTP protocol remains largely invisible, since the respective e-mail program executes it in the background. If the software, the webmail application in the browser, or the mobile mail app don’t automatically define the SMTP protocol when setting up an account, you have to set it up manually to ensure smooth e-mail traffic.