dman3883,

SpamFilter will never send an email out to the internet. All emails, including the NDRs, are forwarded to your destination SMTP server for delivery. In this case, when SpamFilter forwards the NDR to your mail server, your mail server in turns rejects the email with the error logged above after the phrase "The remote server said": "550 unknown user <hidden@thisdomain.com> --"

You will need to ensure that your server allows SpamFilter to relay emails to the internet, in order for them to be delivered.

Please note that SpamFilter allows you to specify a separate SMTP server to be used to deliver NDR emails via these settings in the SpamFilter.ini file:

;An alternate server for sending NDR (non-delivery) notification emails can be used. Leave the "NotificationSMTPServer" value blank to use the default destination SMTP server
NotificationSMTPServer=
NotificationSMTPServerPort=25

Because when SpamFilter rejects an email due to it being spam, SpamFilter, unlike many other products, is not actually sending an email back to the sender (this is a **terrible** practice, as you would be sending an email back to the sender for every spam you receive...). When SpamFilter rejects a spam email, it does so by outputting an SMTP error code during the SMTP transaction. This forces the *remote server* to send out the NDR email back to the sender, not your SpamFilter. Doing so saves a lot of resources, bandwidth, and prevents your network for being blacklisted as a source of spam yourself.

The only times SpamFilter will actually send an NDR is if it encounters problems when delivering clean emails to your server, as in when a user does not exist or when their mailbox is full for example.