4.9 I got a really weird error on startup. It looks like this: “Nov 23 02:10:59 myhost postfix/local[20449]: fatal: unsupported dictionary type: dbm”, what's the problem?

Fortunately this is an easy one. You've used the wrong db type in your /etc/postfix/main.cf. Edit the file and replace every instance of dbm with the word hash. Then issue a postfix reload and you'll be fine.