SMTP bounces can be used in yet another form of Denial Of Service attack. Just imagine what happens when some script kiddie uses a few ten thousand trojaned cable/dsl connected home computers to send email to tens of thousands of domains and they all bounce back to your mail server! Why don't we all just turn SMTP bounces OFF? Like return-receipts, the information content in bounces is very low. A database would be much more efficient if you just want to know wether an email address is spelled correctly. Resending the entire message after adding a few hundred bytes is just idiotic. Escpecially if the attacker only has to send one message to generate 100 bounces. We are currently seeing this first hand: Our real mail.power.net is at 207.151.19.8. The attacker is sending individualized emails with faked headers that contain "mail.power.net (unverified [209.26.14.22])". The recipient computers are dumb enough to send their bounces to the real mail.power.net. This is a DOS because the innocent mail server a) gets millions of bounces and b) might get black listed on various "anti-spam" lists. Dirk Received: from mail.power.net (unverified [209.26.14.22]) by mee.yjapt.co.kr (EMWAC SMTPRS 0.83) with SMTP id <B0000119229@mee.yjapt.co.kr>; Mon, 21 Feb 2000 01:20:18 +0900 Message-ID: <12PAIZTiA2Vyp.5wFyFudzDR_N8@mail.power.net> From: FinancialJobs70972@power.net <FinancialJobs70972@power.net> Bcc: Subject: Private Consultants Needed for Venture Capital Firm Date: Mon, 30 Mar 1998 10:04:48 -0400 (EDT)