From: Martijn de Munnik (martijnyoungguns.nl)
Date: Tue Sep 15 2009 - 05:19:06 CDT

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On Tue, 15 Sep 2009 11:12:52 +0100, Mark Goodge <markgood-stuff.co.uk>
wrote:
> Dan Slay wrote:
>> Thanks, that's what I have read. Which is why this make things more
>> awkward.
>>
>> I cannot see that holding a recipient list is a solution. If, for
>> instance, you relay for thousands of domains all going to different
>> MTA's that hold each individual domains recipient list, its not really
>> that straight forward and may impact performance?
>
> Postfix can generate and maintain an internal recipient list by means of
> a recipient check on the downstream MTA before accepting mail to a
> previously unknown user. See these pages for information on how it works:
>
> http://www.postfix.org/postconf.5.html#reject_unverified_recipient
> http://www.postfix.org/ADDRESS_VERIFICATION_README.html#recipient
>
> Correctly configured, with the right degree of persistance in the cache,
> this will have minimal impact on performance while also significantly
> reducing backscatter. The actual tradeoff between effectiveness and
> performance is dependent on how long data is cached for; if your system
> can cope with it then it's best not to cache at all as that's the only
> way to ensure zero backscatter. But even a fairly lengthy cache time
> will be sufficient to prevent the majority of backscatter, provided that
> the recipient email addresses don't get switched off very often.

One other minor issue with caching is when a mail is received for a
non-existing mailaddress. The cache remembers this. When the mailaddress is
created mail is rejected during the cache period.

>
> Mark

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