SMTP stands for Simple Mail Transfer Protocol. It enables an email client (such as Thunderbird) to deliver messages to other email addresses through a mail server.
SMTP: There Since the Beginning
SMTP has been routing emails around since the earliest days of the Internet! Over time, it has grown stronger with encryption (for safety and privacy) and authentication (anti-spam measures).
While the protocol itself hasn’t changed much in the last several decades, modern mail servers augment or build on top of SMTP with their own filters, protections, and customizations.