Source : Free On-Line Dictionary of Computing

ETRN
     
         ("Extended {TURN}") An {ESMTP} command
        (first defined in {RFC 1985}) with which a {client} asks the
        {server} to deliver queued mail to the client via a new ESMTP
        connection.
     
        ETRN supersedes the {SMTP} "TURN" command in the same way that
        ESMTP's "{EHLO}" supersedes SMTP's "{HELO}".
     
        (1997-11-21)