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Old 11-02-2007, 09:11 AM   #1
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Registered: Nov 2007
Posts: 2

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Using sendmail relay (SMART_HOST) through a spam filter

Good Morning,

I've kind of had support for linux email issues dropped in my lap and this is not really my strong suit.

Here's the situation and I hope I explain it right:

Red Hat Linux server used to generate emails. The client does not use their ISP for mail hosting and their linux server is not public, so their outgoing emails are relayed through the ISP mailserver via SMART_HOST in /etc/mail/


Everything was working great until the ISP installed a spam filter on their mail server.

The out going email generated this error:

----- The following addresses had permanent fatal errors

----- Transcript of session follows -----
550 5.1.2 Host unknown (Name server: host not found)

Content-Type: message/delivery-status

Reporting-MTA: dns;
Arrival-Date: Thu, 1 Nov 2007 15:38:11 -0400

Final-Recipient: RFC822;
Action: failed
Status: 5.1.2
Remote-MTA: DNS;
Last-Attempt-Date: Thu, 1 Nov 2007 15:38:56 -0400

Content-Type: message/rfc822

The ISP provided me with a name and an IP address for the spamfilter.

Adding it to the DNS /etc/resolv.conf didn't help.

Replacing the SMART_HOST entry with it define(`SMART_HOST',`')

regenerating and restarting the sendmail service didn't help either, but I do get a new message:

----- The following addresses had permanent fatal errors -----
(reason: 550 <>: Recipient address rejected: No such domain at this location (

----- Transcript of session follows -----
... while talking to
>>> RCPT To:<>
<<< 550 <>: Recipient address rejected: No such domain at this location (
550 5.1.1 User unknown

Content-Type: message/delivery-status

Thanks for any help you can give me!

Old 11-03-2007, 08:18 PM   #2
Registered: Feb 2005
Distribution: Ubuntu, CentOS
Posts: 585

Rep: Reputation: 35

Does the SPAM filter require that your sendmail log into the ISP's mail server as an authorized client? We had to restrict access to port 25 within our firewall. That means all our Linux servers had to log into our mail server as an authorized client.


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