From: Jonathan Dill (jonathannerds.net)
Date: Thu Apr 17 2008 - 09:52:19 CDT

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On Apr 17, 2008, at 6:13 AM, Aneurin Price wrote:
> Fortunately I only have to forward mail for my organisation's internal
> network of about 50 people, so this isn't a problem for me, but I feel
> slightly less ill-disposed to the ISPs currently sending wads of
> spurious bounces my way, now that I understand that it's not always
> possible to reject rather than bounce if your job is to forward mail.

Ah, since that is the case, hopefully this is obvious and you already
have things set up this way, but one thing you DO want to be sure to
do so that you do not generate backscatter yourself is to make sure
that your relay "knows" about all the valid recipients from the
internal server so that the relay can REJECT mails for invalid
recipients at the relay rather than passing them on to the internal
server and generating a bounce from there. This will also reduce the
load on your relay since rejected messages do not need to be processed
by spam / antivirus stack.

The simplest "low tech" way is to just make a table of valid mail
users, but then if you add / delete accounts you have to remember that
you need to change things in more than one place. If you use e.g.
MySQL or LDAP to store users at the back end, then you could allow the
relay to just check against that.

Jonathan

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