Blocking DSN/NDR Messages Using Outbound Sieve filtering, delivery status notifications (DSNs) can now be blocked from being sent. This can help prevent 'backscatter'.

What is Backscatter

Backscatter is the sending of delivery failure and delivery status notifications back to forged senders. Since spammers typically force the sending address, an MTA will send back delivery notifications to the forged address of the sender.

The recipients of such messages see these as a form of unsolicited bulk e-mail or spam since they were not solicited by the recipients, and can be delivered in bulk quantities. Systems that generate e-mail backscatter can therefore end up being listed on various DNSBLs such as www.backscatterer.org

Filter Details

Use the instructions below to setup a filter on your mail server to prevent DSN messages from being sent.

• Go into MailSite Console and expand Server > Filtering > Sieve Filter > Outbound Relaying Filtering > Server Outbound Script > Advanced. Then paste the rule below:

require "fileinto";

/*
RuleName: Block DSN
RuleDescription:
*/
if
header :contains "Content-Type" "delivery-status" {
fileinto "c:\\DSNBlocks";
}


You may need to add the "fileinto" statement into the require line at the top of the filtering editor.



• Hit OK. This should insert a new rule called Block DSN. You can edit this rule through the GUI if needed.

The rule above will copy these messages to the directory C:\DSNBlocks. You should review the content of this directory to ensure the filter is working correctly. You can then edit the filter through the GUI to reject/discard the messages rather than save these to a directory.

• The following rule can be used to specifically block message delivery confirmation messages rather than all types of DSN's:
  
  require "fileinto";
  
  /*
  RuleName: Block DSN
  RuleDescription:
  */
  if allof (
  header :contains "Content-Type" "delivery-status",
  header :contains "Subject" "Message delivered"
  ) {
  fileinto "c:\\DSNBlocks";
  }