fatal: /etc/postfix/master.cf: line 32: valid hostname or network address required in "[fc00::0.0.0.25]:25"

[Phil Howard]

So it looks like the IP address parser used here doesn't accept all
valid forms of IPv6?

fatal: /etc/postfix/master.cf: line 32: valid hostname or network
address required in "[fc00::0.0.0.25]:25"

It worked when I used "[fc00::0019]:25".