Block E-Mail Bounces with BATV

By Larry Seltzer  |  Posted 2006-07-27 Print this article Print

Opinion: A new standard, implemented in IronPort hardware, can nip blowback in the bud.

Imagine your incoming e-mail volume suddenly leaping 360 times above normal. Its not spam, not strictly speaking. Its a misdirected bounce attack. Bounces used to be a good and useful thing. When you send an e-mail to an invalid address or make some other sort of error, you want to know that it didnt go well. But along the way, bounces got abused just like everything wholesome about e-mail to the point where you had to avoid them as a matter of course.

First, bounces became accomplices to spamming through directory harvest attacks. In this attack, a spammer picks a domain and sends out a large number of messages, guessing at the user name portion of the e-mail address and probably pulling a lot of them out of a directory of names (e.g.,,, etc.). If the spammer gets a bounce on a message, then its not an address in that domain. The messages that dont bounce are real addresses, and then you spam them. Because of this threat, many domains dont send back bounces for wrong addresses anymore.

Another threat these days is what is sometimes called spam blowback. As most of you know, when an e-mail is sent on the Internet from to, there is no mechanism with which the folks at can confirm that the message was in fact sent by, or from anyone at

So imagine that the message is false and not sent by anyone at and that there is no user "recipient" at If still sends bounce messages, it will send them to (if there is such a user), receiving the bounce message, will say to himself, "Huh? I didnt send this."

SMTP authentication standards work may have hit a wall, but the industry has taken the ball and run with it. Click here to read more.

Now imagine that a major phishing attack goes out with millions of e-mails sent from Some percentage of these messages, amounting to a very large absolute number, will be wrong, and the bounces will "blow back" to the mail server at Faceless National.

This has the potential to massively clog the banks infrastructure. According to IronPort, one bounce attack against US Life Insurance increased its inbound mail volume from the typical 10,000 messages to 3,653,201. A jump like that will cause anyone problems. And often the bounce messages themselves will contain malware.

Next page: The answer to blowback.

Larry Seltzer has been writing software for and English about computers ever since—,much to his own amazement—,he graduated from the University of Pennsylvania in 1983.

He was one of the authors of NPL and NPL-R, fourth-generation languages for microcomputers by the now-defunct DeskTop Software Corporation. (Larry is sad to find absolutely no hits on any of these +products on Google.) His work at Desktop Software included programming the UCSD p-System, a virtual machine-based operating system with portable binaries that pre-dated Java by more than 10 years.

For several years, he wrote corporate software for Mathematica Policy Research (they're still in business!) and Chase Econometrics (not so lucky) before being forcibly thrown into the consulting market. He bummed around the Philadelphia consulting and contract-programming scenes for a year or two before taking a job at NSTL (National Software Testing Labs) developing product tests and managing contract testing for the computer industry, governments and publication.

In 1991 Larry moved to Massachusetts to become Technical Director of PC Week Labs (now eWeek Labs). He moved within Ziff Davis to New York in 1994 to run testing at Windows Sources. In 1995, he became Technical Director for Internet product testing at PC Magazine and stayed there till 1998.

Since then, he has been writing for numerous other publications, including Fortune Small Business, Windows 2000 Magazine (now Windows and .NET Magazine), ZDNet and Sam Whitmore's Media Survey.

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