Over the past year we have seen a steady increase in NDR traffic. We’ve done something about it previously but have since gotten far more aggressive on it to the point that virtually every fake bounce will be automatically quarantined.

It’s important to understand the motivation behind the spoofing and massive NDRs they produce. There are two ways in which spammers abuse the NDR system: one is to steal identity and the other is to diminish the confidence in the SPAM filtering solution. The first is quite easy, they want to use a legitimate sender address so that the remote servers will accept the mail. To combat this you can easilly enable SPF/SenderID on your domain and never worry about it. The second is a little more involved/contrived and involves systematically taking apart the ability of the “installed” SPAM filtering solution to adequately sort out mail. Most installed SPAM filtering solutions (the ones you install on your server) and appliances alike (that are devices on your network) build reputation models based on how often legitimate mail comes from certain addresses and IP blocks. They also build local bayesian databases that index known SPAM and non-SPAM; As such, by flooding the server with mail from all over the place those databases the reputation scores become increasingly less reliable – a process more commonly known as poisoning.

So what are we doing and how does it benefit you? Assuming you are using our outbound servers to relay messages, your messages will contain special tracking that will match up what we have in our internal databases. If an NDR is received with that tracking in tact, the message is allowed through. If the NDR is received without that tracking that means that the message didn’t come from you, from your server, that it was spoofed – and it adequately goes into the SPAM quarantine where you’ll likely let it die.