Activating IPv6 support in Ubuntu 16.04 doesn’t have an advantage over keeping it disabled. The benefits of using IPv6, however, are security (though some may argue otherwise) and abundance.

Cheers

There SHOULD be a severe warning

AGAINST activating ipv6

when you run a mail server.

The ipv6 activation alters the name of the server.

This invalidates MX records with the server name.

It took me almost 12 hours to find the real cause of the problem.

My mail server was not working for more than 24 hours.