Thread: RoundCube can only open emails from local users

RoundCube can read emails from other local users, but whenever I try to open a message from an external sender, I get:

Warning: include_once(DOMDocument.php) [function.include-once]: failed to open stream: No such file or directory in /home/vmail/public_html/program/include/iniset.php on line 93

Warning: include_once() [function.include]: Failed opening 'DOMDocument.php' for inclusion (include_path='/home/vmail/public_html/:/home/vmail/public_html/program:/home/vmail/public_html/program/lib:/home/vmail/public_html/program/include:.:/usr/share/php5:/usr/share/php') in /home/vmail/public_html/program/include/iniset.php on line 93

Fatal error: Class 'DOMDocument' not found in /home/vmail/public_html/program/lib/washtml.php on line 235

And a message at the top saying the operation timed out.

Any ideas? What logs might I look at?
Thanks