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Old 06-05-2009, 12:29 PM   #1
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sendmail error 550 - Host Unknown

I'm new to Linux and need help on sendmail. We have a linux box where I have created some scripts. However, a functionality I need is to be able to send email using another smtp server.
my linux box address is and my smtp server (windows) is

This is what I have in my
# "smart" relay host (may be null)

When I type the command
echo this is a test | /usr/sbin/sendmail

I get the error:
550 5.1.2 Host Unknown (name server: host not found)

I'm able to ping the server and the smtp server is setup to allow relays from this box

Any ideas?

Thank you for your help
Old 06-05-2009, 06:58 PM   #2
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You should put the IP address in square brackets
to avoid nslookups. If that doesn't work you should add an entry in /etc/hosts (or in your dns server, if you use one) for, so sendmail can resolve it.
Old 06-08-2009, 09:33 AM   #3
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Smile sendmail error 550 - Host Unknown

Thanks for the suggestions. I had already put the IP in the /etc/hosts.

I found the solution. This is what happened:

When I read the Redhat documentation, it said that I needed to created the in the /etc/mail folder. That's what I did. Apparently the changes I made never worked because the system was looking at the /etc/ folder and not /etc/mail folder for the So just to give it a try, I re-created the in the /etc/ folder and it worked. The problem was then with the documentation.

Thanks again.


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