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NDR for unknown user not sent

Printed From: LogSat Software
Category: Spam Filter ISP
Forum Name: Spam Filter ISP Support
Forum Description: General support for Spam Filter ISP
URL: https://www.logsat.com/spamfilter/forums/forum_posts.asp?TID=6552
Printed Date: 15 March 2025 at 4:33pm


Topic: NDR for unknown user not sent
Posted By: dman3883
Subject: NDR for unknown user not sent
Date 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  mailto:redacted@somedomain.com - redacted@somedomain.com to  mailto:hidden@thisdomain.com - 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: mailto:redacted@somedomain.com - redacted@somedomain.com to: mailto:hidden@thisdomain.com - hidden@thisdomain.com was returned to sender - The following recipients had delivery problems: mailto:hidden@thisdomain.com - hidden@thisdomain.com --  --  --  -- The remote server said: -- 550 unknown user < mailto:hidden@thisdomain.com - hidden@thisdomain.com > --
 
(actual addresses have been removed from log)
 
Anyone have any idea why? Thanks for the help.


Replies:
Posted By: LogSat
Date 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 < mailto:hidden@thisdomain.com - 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


-------------
Roberto Franceschetti

http://www.logsat.com" rel="nofollow - LogSat Software

http://www.logsat.com/sfi-spam-filter.asp" rel="nofollow - Spam Filter ISP

Posted By: dman3883
Date 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?

Posted By: LogSat
Date 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.

-------------
Roberto Franceschetti

http://www.logsat.com" rel="nofollow - LogSat Software

http://www.logsat.com/sfi-spam-filter.asp" rel="nofollow - Spam Filter ISP


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