SpamFilter will never send an email back to the sender when an email is rejected. Instead, SpamFilter will reject spam emails with an SMTP error message. This causes the remote server to send an NDR email to their customer (the sender) to notify them the email was rejected. Please note that, when an email is blocked, the sender *must* be notified to inform them of the delivery problems. If not, the sender will not know an email was blocked, and the recipient will never see the email unless they look in their quarantine (but this defeats the purpose of a spamfilter, if we require users to continuously check their quarantined emails anyways...).

This said, if admins are absolutely certain they do not want SpamFilter to cause andy NDR emails back to the sender, the following option is available in the SpamFilter.ini file to prevent this:

;Setting DoNotSendNDROnQuarantine to 1 will prevent generation of NDR when email are quarantined by causing SpamFilter *not* to send an error code when quarantining emails

DoNotSendNDROnQuarantine=0


Please do note that allowing SpamFilter to take care of the NDRs will prevent any email notifications from leaving your network. There are many other anti-spam products that instead of behaving as SpamFilter does, will actually send back an email with the NDR back to the sender (which in 90% of the times is an innocent bystander and/or a non-existant address). This will often cause the anti-spam server itself to be blacklisted as there will be tons of spam originating from it in the form of NDR emails.

Handling your own NDRs by sending bounce emails to the senders may cause you the same problems, so proceed with caution to minimize the risk of sending emails to invalid/innocent emails back out to the internet.