The technology that is essential for email delivery is the Simple Mail Transfer Protocol (SMTP). SMTP is a protocol used for sending and routing email messages between servers on the internet. It outlines the rules for how emails are transmitted from the sender's server to the recipient's server. Alongside SMTP, other protocols like POP3 or IMAP are used for retrieving emails from a server, but SMTP specifically handles the outgoing messages. This fundamental technology ensures effective communication through email.