general

They're both mail commands and function as follows:

Turn- An SMTP command with which a client
asks the server to open an SMTP connection
to the client, thus reversing their roles.

this command has been superceded by

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 supercedes the SMTP "TURN" command in the
same way that ESMTP's "EHLO" supercedes SMTP's "HELO".

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