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I have server with RHEL 6 and postfix (postfix-2.6.6-2.2.el6), When I try to send email to Google I'm getting this in /var/logmaillog:

postfix/smtp[13414]: 6F4DC10406E7: to=<>, relay=none, delay=0.16, delays=0.02/0/0.14/0, dsn=4.4.1, status=deferred (connect to[2a00:1450:4013:c01::1b]:25: Network is unreachable)

This error occurs only on this domain (

What does it mean connect to Network is unreachable?

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This means your MTA is trying to connect to the Google SMTP via IPv6 (2a00:1450:4013:c01::1b). This doesn't work as your network probably doesn't support IPv6.

To get rid of the error message, you can configure Postfix not to use IPv6 by editing your /etc/postfix/ with this directive:

inet_protocols = ipv4

Afterwards you'll have to restart postfix:

/etc/init.d/postfix restart
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It probably means that there is some issue with his name resolution, or routing to the IPv4 MX also, as unless you configure postfix as IPv6 only, it will always try the IPv4 answers, then the IPv6 responses. – NickW Mar 28 '13 at 9:45
To be able to tell that, one would need to see the rest of the log. The provided line only tells that IPv6 connection is not working. Perhaps the mail is delivered via IPv4 in the next line. As far as I know, postfix will per default try IPv6 first, then IPv4 as fallback: – etagenklo Mar 28 '13 at 10:44
You're right, that has changed, as specified here: – NickW Mar 28 '13 at 10:50
This won't help, when the problem really is that the server isn't connected to the Internet. – Michael Hampton Mar 28 '13 at 14:07

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