[OT] How to avoid Backscatter in Sendmail

Kevin Miller Kevin_Miller at ci.juneau.ak.us
Wed Apr 21 17:56:05 IST 2010

Marcello Anderlini wrote:
> I beg your pardon but for this OT but I'm desperate, my system is
> blacklisted by Backscatterer.org and I'm trying to configure it to
> avoid backscatter.  
> I'm following what suggest here:
> http://elqui.dcsc.utfsm.cl/util/email/backscatter.html
> I'm using sendmail-8.13.1-3.3.el4. on a CentOS release 4.8.
> I configured my access file to reject unknown recipients but my
> system still sends an email instead of rejecting it at smtp initial
> transaction.  
> I would be very grateful for any kind of help anyone could give me.

Your backscatter may not be coming from you.  It's quite likely that your users have been 'joe-jobbed' and some other mail server is bouncing spam.  Since the From: field is forged with your domain, it could look like it came from you to a mail admin that's new to the game.

Couple of obvious things - did you remake the /etc/sendmail.cf file?  The m4 command to do so is given at the top of the sendmail.mc you posted.

Also, did you remember to hash your access file?  That's bitten me once or twice before: I edited /etc/access then forget to do 'makemap hash access < access' afterwards.  

Another thing you could look into if you're not already using it is SPF.  It helps cut down on a lot of forged email.

Finally, if your sendmail server is just a relay pointing to an internal server, look into smf-sav or milter-ahead.


Kevin Miller                Registered Linux User No: 307357
CBJ MIS Dept.               Network Systems Admin., Mail Admin.
155 South Seward Street     ph: (907) 586-0242
Juneau, Alaska 99801        fax: (907 586-4500

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