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Old 6th June 2009, 12:04
countrydj countrydj is offline
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Default Why do I get 550 5.1.1 Usaer unknown message ??

I am using Centos 5 on my linux server.

This is my problem:
Using my own server's SMTP (Relaying is allowed from my ip number),if I send an email, using Eudora or any other mail client, to one of my customers it will not send. It gives a 550 5.1.1 User unknown message.
I can send email to any of my other clients without problem.

Using my ISP smtp server, the same email will send and is received by the client.

There is obviously something missing on my server that is refusing to accept this particular email address.

Can anybody suggest anything that may cause this to happen???


John C
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Old 7th June 2009, 18:25
falko falko is offline
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Can you post an excerpt of your mail log when this happens?
Which MTA are you using? Postfix or Sendmail?
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Old 7th June 2009, 19:41
countrydj countrydj is offline
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Hi Falko...
Many thanks for your replied to this problem.

I have now solved the problem:

hisdomain1.com was listed in local-host-names

The domain is not hosted on the sending server.
It seems that the sending server checked and found hisdomain1.com was listed in local-host-names, but could not find the user on the server, so returned a 550 5.1.1 error.

Since hisdomain1.com was listed in local-host-names
on the server that hosts the domain, there was no problem with any other sender getting mail through.

Hope this makes sense...

Thanks for your help

John C
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Old 18th January 2010, 02:10
mroradba mroradba is offline
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Default 550 5.1.1 ... User unknown

Hello, I think I have run into simmillar issue and can't solve it right now. I try to use mailx to send an e-mail message from Centos (root) using one of the valid mail accounts I have on the mail server (can be pinged and nslookup works fine).

However I still have same issue with ... User unknown:

[root@centos01 mail]# mailx -v mailadmin@acx.local
Subject: test mail
Null message body; hope that's ok
mailadmin@acx.local... Connecting to [] via relay...
220 centos01.acx.local ESMTP Sendmail 8.13.8/8.13.8; Sun, 27 Dec 2009 14:41:18 +0100
>>> EHLO centos01.acx.local
250-centos01.acx.local Hello localhost.localdomain [], pleased to meet you
250 HELP
>>> MAIL From:<root@centos01.acx.local> SIZE=44 AUTH=root@centos01.acx.local
250 2.1.0 <root@centos01.acx.local>... Sender ok
>>> RCPT To:<mailadmin@acx.local>
>>> DATA
550 5.1.1 <mailadmin@acx.local>... User unknown
503 5.0.0 Need RCPT (recipient)
>>> RSET
250 2.0.0 Reset state
>>> RSET
250 2.0.0 Reset state
root... Using cached ESMTP connection to [] via relay...
>>> MAIL From:<> SIZE=1068
250 2.1.0 <>... Sender ok
>>> RCPT To:<root@centos01.acx.local>
>>> DATA
250 2.1.5 <root@centos01.acx.local>... Recipient ok
354 Enter mail, end with "." on a line by itself
>>> .
250 2.0.0 nBRDfIJl004436 Message accepted for delivery
root... Sent (nBRDfIJl004436 Message accepted for delivery)
Closing connection to []
>>> QUIT
221 2.0.0 centos01.acx.local closing connection
You have new mail in /var/spool/mail/root

Could someone please advise?

Thank you,
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Old 18th January 2010, 02:23
mroradba mroradba is offline
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Ok it looks like it worked. I have completely removed entries from local-host-names and restarted sendmail.

Since I use this for a test configuration and acx.local is my only test domain it worked perfectly with removing other entires.

This configuration is only for using mailx as a client, not as a server. I have mail server on another machine.

Hope this will help somebody.

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