Spam Filter ISP Forums : NDR for unknown user not sent

NDR for unknown user not sent : Because when SpamFilter rejects...

Wed, 10 Sep 2008 16:43:00 +0000

Author: LogSat
Subject: 6552
Posted: 10 September 2008 at 4:43pm

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.

NDR for unknown user not sent : Ok I'll check it out. But...

Wed, 10 Sep 2008 14:19:55 +0000

Author: dman3883
Subject: 6552
Posted: 10 September 2008 at 2:19pm

Ok I'll check it out. But why would all the other NDR's be delivered, like rejections for blacklisting, just not the NDR for an unknown user?

NDR for unknown user not sent : dman3883,SpamFilter will never...

Tue, 09 Sep 2008 21:47:49 +0000

Author: LogSat
Subject: 6552
Posted: 09 September 2008 at 9:47pm

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

NDR for unknown user not sent : When someone sends to an email...

Tue, 09 Sep 2008 17:35:34 +0000

Author: dman3883
Subject: 6552
Posted: 09 September 2008 at 5:35pm

When someone sends to an email address that doesn't exist on our mail server, the unknown user NDR is not delivered back to the sender. All other NDR's for rejected emails that trigger the filters are sent.

I can see in the log files that spamfilter is claiming to generate the NDR for the unknown user, but the sender doesn't get it.

09/09/08 14:03:06:593 -- (2088) EMail from redacted@somedomain.com to hidden@thisdomain.com -- was forwarded to ***.***.***.***:25
09/09/08 14:03:06:593 -- (2088) Some recipients do not exist, sending NDR bounce to sender
09/09/08 14:03:06:593 -- (2088) EMail from: redacted@somedomain.com to: hidden@thisdomain.com was returned to sender - The following recipients had delivery problems: hidden@thisdomain.com -- -- -- -- The remote server said: -- 550 unknown user <hidden@thisdomain.com> --

(actual addresses have been removed from log)

Anyone have any idea why? Thanks for the help.