I recently discovered that external monitoring services like siteuptime.com may not pass HTTP headers identifying the request as originating from a browser. This can cause mail_digests.php to fail in these situations. They occur if you have no cron or scheduled task capability on your web host and your web server is configured to not allow traffic which does not appear to be from a browser.
If you want, I can ping mail_digests.php on your server for a $20 setup fee and $5/month for calling one instance hourly. Contact me if interested.