If you get this error when accessing /webmail:
ERROR (512): Smarty error: problem creating directory "tmp/_cached_templates" (/webmail/smarty/Smarty.class.php:589)
It likely means that you've enabled safemode globally in the /usr/local/lib/php.ini.
If you want to use safemode, do so on a per-domain basis by going to:
Admin Level -> SafeMode Configuration or change manually in php.ini