ETRN (Extended Turn) is an extension to the Simple Mail Transfer Protocol (SMTP) that allows an SMTP server to send a request to another SMTP server to send any e-mail messages it has. Typically, SMTP is used with two other protocols, Post Office Protocol 3 (POP3) or Internet Message Access Protocol (IMAP), to request messages from a server because SMTP by itself cannot request mail to be sent.
ETRN is designed for use by anyone who is traveling and wants to get access to their mail. ETRN can only be used with Internet service providers that support ETRN. The latest version of the widely used SMTP server, sendmail, supports ETRN. The details of ETRN are in Request for Comments 1985 of the Internet Engineering Task Force (IETF), SMTP Service Extension for Remote Message Queue Starting.