Are “Non-deliverable Reports” (NDRs) returned when the spam filter blocks the delivery of a message?
The spam firewall is configured to not send back an NDR in response to Blocked messages. Most Blocked messages are blocked as a result of the vendor maintained Blacklist, and are Blocked during the SMTP conversation; the message is not accepted so no NDR should be sent by the spam filter. The originator may still receive an NDR from the sending mail server but not from the spam firewall. The spam firewall does not send an NDR in response to messages that are blocked because most spam is “from” a forged sender; sending an NDR message in response to what is blocked ends up “spamming” the forged sender’s email address. The sender didn’t actually send the message initially, but would end up receiving the NDR.
