1. Introduction

   The TURN command was a valid attempt to address the problem of having
   to start the processing for the mail queue on a remote machine.
   However, the TURN command presents a large security loophole.  As
   there is no verification of the remote host name, the TURN command
   could be used by a rogue system to download the mail for a site other
   than itself.

   Therefore, this memo introduces the ETRN command.  This command uses
   the mechanism defined in [4] to define extensions to the SMTP service
   whereby a client ("sender-SMTP") may request that the server
   ("receiver-SMTP") start the processing of its mail queues for
   messages that are waiting at the server for the client machine.  If
   any messages are at the server for the client, then the server should
   create a new SMTP session and send the messages at that time.