more details in the config.inc.php:
$config['password_algorithm'] = "blf-crypt";
$config['password_algorithm_options'] = "blf-crypt";
$config['password_algorithm_prefix'] = "{BCRYPT}";
$config['password_dovecotpw'] = "/usr/local/bin/doveadm pw";
$config['password_dovecotpw_method'] = "BLF-CRYPT";
$config['password_dovecotpw_with_method'] = true;