[Spambayes] : Stopping spam at SMTP Level

Christopher Jastram cej at intech.com
Mon Feb 9 23:02:02 EST 2004

Terrel Shumway wrote:

> Christopher Jastram wrote:
>> Well, I've seen one solution that I really really like.  It works 
>> like this: mail is handled by a third party.  You sign up for an 
>> email address from that party, and they give you one for $20/year or 
>> so.  
> Why is this necessary? 5000 users*$20/year = $100,000/year.  Nice 
> pocket change.  Would you like to make a donation to my favorite 
> charity? 8-) 

Nah...  I've just seen the commercial offerings, and they range from 
$5/year to $20/year.  I've considered signing up for my own use, but 
then I say -- "why not start my own??"  As if I didn't have enough to do 
already!  :)

>> Everyone who sends an email gets a bounce saying "Please follow this 
>> link and answer the question to send mail to this person."  At the 
>> link you will find a simple question like: "Choose the red square" or 
>> "one plus one equals ?".  Answering the question adds the sender to 
>> the database of "humans," and mail will be allowed from that 
>> address.  Kinda neat, and it will be what I set up eventually.
> This addresses a very small part of the problem with a very expensive 
> (usability-wise) solution.

True.  It's also easy to foil.  The only reason it works is because the 
mail-proxy sites remain fairly small.

>> The best idea I've seen is RBL.  (Realtime Blackhole List)  An RBL is 
>> a list of known spam-sending networks.  Administrators subscribed to 
>> an RBL agree to completely drop all traffic originating from or going 
>> to said spam-sending networks.   Nice system, and it works quite well 
>> because ISPs realize that it hurts their business to allow spam on 
>> their networks.  Unfortunately, one must have a very flexible and 
>> understanding boss to pull this one off, and not many IT 
>> administrators have that luxury.
> RBL, of course, also has its drawbacks, which have been thoroughly 
> discussed elsewhere.  The two-camp approach is a good evolution of 
> RBLs, but won't help us today.
>>>> My solution works like this:
>>>> 1) Postfix accepts the mail, checks to see if it's
>>>> sent to a valid user
>>>> 2) If it is, run it through spambayes via
>>>> content_filter, which re-injects the mail into the system.  That 
>>>> "run it
>>>> through spambayes" script looks at the "to: " mail header and uses the
>>>> appropriate user-specific database accordingly.
>>>> 3) Postfix hands it off to Cyrus, which delivers via
>>>> POP3 or IMAP.
> Using spambayes (step 2) on the wire (i.e. instead of step 1)  may not 
> save bandwidth, but can save disk space and give priority to non-spam.
>    1) a message looks like spam: 553 it and you're done. Include a URL 
> in the response text so a human can get whitelisted and resend a false 
> positive.    2) If a message is "unsure", 553 it but store it for 7 
> days so the human user can redeem it from quarantine without resending 
> it.
>    3) tar-pit the spam-sending IP/network so it will take them three 
> hours to send a single message.
> Now you have a good 80% solution that will save your CPU and push ham 
> to the front of the queue.

Good ideas.  Thank you very much -- I've been racking my brain for 
ideas, and input is much appreciated.

>>>> A lot of the spam we get is bounces from remote mail
>>>> servers.  Spammers spoof our domain, and we get the "invalid-user"
>>>> bounces.  Sick.  I've been just discarding everything that's from
>>>> mailer-daemon and not to a valid local user.
> not a bad idea.

Thanks!  It works quite well.  Delivery works out to about 400 messages 
/ day with this system, and I'm dropping 30,000-35,000 messages without 
processing.  Nobody has complained, and everybody gets their daily quota 
of spam.  I'm thinking of re-integrating the spambayes filtering, but 
that will have to wait (busy teaching Intro to VBA this week).

Is there some web site that has tips for battling spam?  Tried-and-true 
practices gleaned from bitter mouths of hard-pressed sysadmins?


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