How to Stop NDR Spammers

Recently Asked Question:  Can Mailspect stop NDR spammers?

The antispam business is a constantly evolving game of cat and mouse.  No one can afford to under-estimate the creativity of the criminal mind.  Spammers are constantly developing new techniques to evade and fool spam filters and engines.  Recently, they have developed a clever way of circumventing spam filters called NDR.

The technique exploits so-called NDR emails which stands for Non-Delivery Reports.  This is a standard for the SMTP protocol.  Most of us call these emails “bounce back” notices.  NDR emails  are normally triggered by the user entering an incorrect email address that the email server cannot deliver.   They have also been called Non-Delivery Receipt, a (failed) Delivery Status Notification (DSN) message, and a Non-Delivery Notification (NDN).  The spammers send emails masquerading as NDRs and because they look legitimate they slip through the real-time black lists and spam engines such as Spamassassin, Cloudmark and Mailshell.

Mailspect has developed robust techniques based on content filtering and rate limiting to combat and stop NDRs.  We think that it is the best NDR spam defense on the market.  The actual techniques involved are viewed as trade secrets.  They are disclosed to customers and well-known participants in the Open Source community to avoid the techniques being analyzed and perhaps circumvented by professional spammers.

If you would like to understand Mailspect’s NDR defense techniques in the context of a 30-day trial of Mailspect Defense, please contact


Stay in touch with the latest MailSpect News and Case Studies.