Dovecot: auth: Fatal: Unknown passdb driver ‘thesql’

On CentOS 6 is necessary to install not only the dovecot package, but also the additional modules, in this case the package dovecot-pgsql is also needed for postgresql support.

The right syntax for the driver line in /etc/dovecot/conf.d/auth-sql.conf.ext is:

driver = sql

and then the right driver is specified in /etc/dovecot/conf.d/dovecot-sql.conf.ext

driver = pgsql

Based on your edits I don't see that you have configured Dovecot's auth socket listener at all. It is number 34 and 35 of the Dovecot section in the guide you linked.

If you are using the conf.d directory structure (as suggested in the guide) make sure all the relevant sections like conf.d/10-master.conf are included and where they are supposed to be.

After you fixed or added these, restart Dovecot and doveconf -n should contain the following somewhere:

...
  unix_listener /var/spool/postfix/private/auth {
    mode = 0666
    user = postfix
    group = postfix
  }
...

You don't need to open any special ports on you firewall since Postfix and Dovecot are on the same machine and Postfix talks to Dovecot via the socket listener the above directives configure.