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From: Michael Tokarev (mjt_at_tls.msk.ru)
Date: Wed Oct 02 2002 - 12:05:37 CDT

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    Niclas Söderlund wrote:
    > Hiya folks!
    >
    > Im using postfix as SMTP (courier as pop3 and maildrop as deliveryboy to my
    > Maildir's) and it works excellent!
    >
    > Now, I was searching for a howto or alike to be able to setup ETRN support
    > in my mailserver. I have two backup mailservers that have higher MX-records
    > than my primary server, and when (and if) that goes down, I need those
    > backup mailservers to deliver the mail to the primary mailserver when its
    > back online. And that I would like to do thru ETRN. But it tells me its "not
    > implemented".

    This setup - backup mx - is a common way of doing things (but try to avoid
    having backup MXes if at all possible, because of various reasons). Almost
    every real mailserver does this function in a right way - it will try to
    deliver mail to your server *automatically*, without a need for ETRN *at all*.
    ETRN may be needed when your primary MX goes down on a regular basis, or is
    unavailable most of the time. Usually, backup MX server will try to send
    mail to primary at regular intervals, until that will succed or mail will
    sit in queue for too long time. ETRN may trigger delivery at any given
    time, and this may be useful when you want to see all your mail *immediately*
    when your primary goes up, but this usually isn't a real requiriment, it's
    ok to wait for a hour for a backup MX to deliver your mail itself.

    Concerning your backup MX servers - talk with their admins about support
    of ETRN command. Postfix does supports ETRN, but it's on your primary,
    and we're talking about your secondaries that doesn't run postfix.

    And for ETRN itself - you need to write simple shell/perl/whatever script
    that will be executed when your machine is booted and will send ETRN
    command to your secondaries. Netcat package maybe useful here -- just
    "HELLO yourhost", "ETRN yourdomain", "QUIT" - that's all.

    /mjt

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