prova così:

<VirtualHost webmail.keyworld.it:80>
DocumentRoot /var/www/webmail
ServerName webmail.keyworld.it

ScriptAlias /cgi-bin/ /var/www/webmail/cgi-bin/openwebmail/
<Directory "/var/www/webmail/cgi-bin/openwebmail/">
AllowOverride All
Options +ExecCGI
Order allow,deny
Allow from all
</Directory>

</VirtualHost>