This morning, from about 7:00am to 10:30am, access to the Internet was broken. The visible symptoms included "page not found" for external websites, or extremely slow access at best. Incoming and outgoing email messages to non-Exchange users were affected as well. Access to internal web pages was not affected.
The problem was eventually isolated to an intermittent fault with the Fortinet content filtering engine. The appliance was reset, after which normal operation resumed at about 10:15am.