Since adding IPv6 capabilities to our spam scanning system, we have had a handful of issues with customers running Postfix mail servers. In some versions of Postfix, the software will try to get a quad A records (AAAA) whether or not the mail server has IPv6 connectivity. This error will be highlighted by messages like:

Network Is Unreachable

There are a couple of options, but the easiest option is to force Postfix to run in IPv4 only by editing the /etc/postfix/mail.cf and ensuring the line starting with inet_protocols is:

inet_protocols=ipv4

This will ensure the system only asks for ipv4 (A) records and should resolve the issue.