The Free On-line Dictionary of Computing (30 December 2018):

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)