Re: Kolab-users Digest, Vol 46, Issue 15

> Subject:
> TO all anti-spam enthusiasts
> From:
> "Alain Spineux" <aspineuxgmail.com>
> Date:
> Thu, 13 Dec 2007 23:34:31 +0100
> To:
> "Kolab development coordination"
<kolab-develkolab.org>, "Kolab 
> Users" <kolab-userskolab.org>
>
> To:
> "Kolab development coordination"
<kolab-develkolab.org>, "Kolab 
> Users" <kolab-userskolab.org>
>
>
> Hi
>
> Recently, I saw a lot of post about activating some
"postfix antispam" features.
>
> I would like to react!
>
> The first goal of an email system is to exchange
messages between peoples.
> Anything that prevent messages to pass through must be
discuss with care!
> [...]
> Only grey and black listing are "dynamic"
measures against SPAM that
> SPAMMER cannot control!
>
> Regards
>   
I couldn't agree more.

I recently implemented greylisting for one of our email
domains, and the 
results were amazing! Of 11041 emails received in the
following 24 
hours, 40 were actually delivered. The remaining 1101 did
not retry!

I am now looking for a more sophisticated  greylisting
implementation 
which would allow me to recognise some further spamming
behaviour in a 
fully dynamic way.

For instance, recognising multiple emails from the same IP
address to 
different invalid addresses on our domain.

In all other areas of my firewall, I also only implement
dynamic 
blocking, so that suspicious behaviour is blocked
temporarily, which 
protects my network, but then the block is removed, so that
I don't 
later block potentially correct traffic.

Cheers!
Nik

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>> Hi
>>
>> Recently, I saw a lot of post about activating some
"postfix antispam" features.
>>
>> I would like to react!
>>
>> The first goal of an email system is to exchange
messages between peoples.
>> Anything that prevent messages to pass through must
be discuss with care!
>> [...]
>> Only grey and black listing are "dynamic"
measures against SPAM that
>> SPAMMER cannot control!
>>
>> Regards
>>   

>>> Nik777 <kolabbabel.homelinux.net>
20.12.2007 02:20 >>>
> I couldn't agree more.
>
> I recently implemented greylisting for one of our email
domains, and the 
> results were amazing! Of 11041 emails received in the
following 24 
> hours, 40 were actually delivered. The remaining 1101
did not retry!
> 
> I am now looking for a more sophisticated  greylisting
implementation 
> which would allow me to recognise some further spamming
behaviour in a 
> fully dynamic way.
> 
> For instance, recognising multiple emails from the same
IP address to 
> different invalid addresses on our domain.
> 
> In all other areas of my firewall, I also only
implement dynamic 
> blocking, so that suspicious behaviour is blocked
temporarily, which 
> protects my network, but then the block is removed, so
that I don't 
> later block potentially correct traffic.
> 
> Cheers!
> Nik

Checking spam with rbl lists is pretty effective, too. I got
several domains which recieve 25000 spam mails per day and
more. Because of my rbl list I can block up to 99% of this
spam before it gets queued.
Rbl lists are dynamic, very easy to integrate in postfix and
cost one dns query per mail and rbl list. 

This is my "smtpd_recipient_restrictions" in
postfixs main.cf (non kolab).

smtpd_recipient_restrictions =
        permit_sasl_authenticated
        permit_mynetworks
        reject_unauth_destination
        reject_invalid_hostname
        reject_non_fqdn_sender
        reject_unknown_sender_domain
        reject_non_fqdn_recipient
        reject_unknown_recipient_domain
        reject_rbl_client zen.spamhaus.org
        reject_rbl_client bl.spamcop.net
        reject_rbl_client cbl.abuseat.org
        reject_rbl_client dnsbl.ahbl.org
        reject_rbl_client dialups.mail-abuse.org
        reject_rbl_client dynablock.sorbs.net
        reject_rbl_client relays.mail-abuse.org
        reject_rbl_client list.dsbl.org
        permit


Simon

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