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	Mon, 12 May 1997 07:51:39 +0800 (WST)
Date: Mon, 12 May 1997 07:51:39 +0800 (WST)
From: Adrian Chadd <adrian@obiwan.psinet.net.au>
To: Dev Chanchani <dev@wopr.inetu.net>
cc: freebsd-isp@FreeBSD.ORG
Subject: Re: Sendmail hack
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> I and my users have been receiving a lot of spam mail recently. Usually,
> this is not so bad because I will just reply to remove me. However, more
> and more spammers are using fake domain names to send their spam from. I
> was wondering if you could hack sendmail to do a lookup on the reply to or
> from address. If the domain name is non-existant, reject the mail. 

[snip]

Info to do this is freely avaliable on the internet, however just add this
to the end of your sendmail.cf file :


Scheck_mail
# Check for valid domain name
R$*				$: <?> $>3 $1
R<?> $* < @ $+ . >		$: <OK>
R<?> $* < @ $+ >		$#error $: 451 Domain must resolve

Note this is incompatible with the DeliveryMode=defer option, since it
requires that a DNS lookup be done immediately upon mail receipt.

Adrian.