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# The Answer Gang

#### By Jim Dennis, Ben Okopnik, Dan Wilder, Breen, Chris, and... (meet the Gang) ... the Editors of Linux Gazette... and You!
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By Dan Wilder

> I've been filtering more agressively. This month's spam bouncing features a 450 to "From: " addresses with domains the MTA can't find in the DNS.

> Since lots of spammers use unrepliable "From:" this knocks those guys off. We use a 450 instead of a 550 because a 450 is retryable. If it's just a DNS glitch, the retry goes through.

> So the real slimebags use a nonexistant user at a host that exists but for some reason does not accept SMTP connections. That way they pass the "does this host exist" test.

> Lots of their mail goes to 10000000 VERIFIED EMAIL ADDRESSES, meaning people like mlr@ssc.com who don't work here any more, or Qr457.1121212???@ssc.com who never did. These bounce, but our MTA can't raise a connection from the putative source, so they just park on the queue for a week or so.

> Every couple of days I go look for new stuff on the queue from MAILER-DAEMON with "Connection refused" errors. Then I add them to a reject file, and henceforth mail claiming to be from anybody at the "From:" domain gets

>     550 You refuse our connections so we refuse yours

> Non-retryable. I figure we don't knock off too many legit domains, since these usually don't keep refusing connections for very long, and MAILER-DAEMON doesn't have much traffic for legit domains anyway.

> Each morning I get mailed the list of 100 or so mails that were so refused, and I vette for stuff that might be legit. Mostly it's the same bogus "From:" hammering ten or twenty users at SSC, more than half of whom never existed.

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Published in issue 76 of Linux Gazette March 2002

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