If you want to change the webmail cache type follow this steps.
Path /usr/share/psa-horde/lib/Horde/Cache
The available cache methods
Config path
/usr/share/psa-horde/lib/Horde/config.php
change the $conf[‘cache’][‘driver’] = ‘sql ‘; to $conf[‘cache’][‘driver’] = ‘apc’;
Log Files path
/var/log/psa-horde