What does it mean connect to aspmx.l.google.com:25: Network is unreachable?

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=<[email protected]>, relay=none, delay=0.16, delays=0.02/0/0.14/0, dsn=4.4.1, status=deferred (connect to aspmx.l.google.com[2a00:1450:4013:c01::1b]:25: Network is unreachable)

This error occurs only on this domain (englishonlineuniversity.com).

What does it mean connect to aspmx.l.google.com:25: Network is unreachable?

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/main.cf with this directive:

inet_protocols = ipv4

Afterwards you'll have to restart postfix:

/etc/init.d/postfix restart

Some ISPs don't allow localhost as a domain name (QFDN) and will reject incoming email from your server. In /etc/postfix/main.cf alter the following:

myhostname = localhost

to a fully qualified domain name you own, i.e:

myhostname = geoip-db.com
mydestination = geoip-db.com, localhost.localdomain, localhost