Qmail is executing the script as the mailbox owner. Postfix can probably be made to do it, but changing that definitely requires root access and a look in the Postfix docs.

Your script is probably running as 'nobody' or a similar non-privileged account.

A work around:
1. The email script writes files in a public directory: /tmp/news-update or similar.
2. A cron job is run every 1-5 minutes, as the user who owns the web directory, it reads /tmp/news-updates and incorporate changes.