My email server now functions on both IPV4 and IPV6 networks. Postfix is IPV6 ready but, the configuration needed some adjustments.
In places like mynetworks, found in main.cf, IPV6 addresses must be enclosed in square brackets. This does not include the netmask. For example: [::1]/128
In main.cf Postfix is told to use IPV6 using inet_protocols. Set this to all and Postfix will listen to IPV4 and IPV6 interfaces.
For more information about Postfix and IPV6 see the Postfix web page
In fact a lot of configuration files have special rules to accommodate IPV6 addresses because IPV6 syntax was not a consideration when the configuration syntax was first invented. Read configuration documentation carefully.