According to Slashdot, Google’s mail servers appear to be responsible for sending large amounts of backscatter. They don’t perform any recipient validation for the googlegroups and blogger.com domains (and presumably their other domains as well), allowing spammers to launch large-scale dictionary attacks against them using forged headers and envelope sender addresses. This results in the owners of those forged addresses getting huge amounts of bounce messages when the spam hits non-existent users on Google’s domains. Most correctly set up mail servers don’t generate such bounce messages. Tell that to Google’s mail server! Botnets love mail servers like this and will go to town on them, commencing an unrelenting barrage of spam.

Most ISPs won’t hesitate to place a block on any IP that receives complaints of backscatter, and that can cause big headaches for innocent people. There are even reports of businesses having entire mail servers wiped out due to backscatter.

What Google should be doing is rejecting traffic to bogus users during the SMTP transaction. Several techniques can be used to do this:

• Recipient validation
• Reject senders on dynamic black lists
• Reject. email from servers senders that do not have a reverse DNS entry

Unfortunately Google is doing none of them. Slashdot also reports that emails sent to abuse@google.com and postmaster@google.com went unanswered except for a canned response that didn’t address the situation.

It’s very surprising that Google, whose Gmail program has been widely praised for its spam controls, would have such badly misconfigured mail servers. Ironically, those same spam controls have reportedly been blacklisting Google themselves. According to an article on newswireless.net, Gmail placed a user’s Google Alerts in his spam folder. Ah that wacky Google!

For more information, the website DontBounceSpam.org has an extensive list of resources and tips for server admins and end users on how to fight backscatter and reduce overall spam.