This post discuss the doubts about the automatic time-out, or session length, feature of Microsoft Office Outlook Web Access (OWA). Sometimes people are getting unexpectedly bounced out of OWA sessions, and sometimes they’re just worried that other customers are using OWA without being aware of the potential security issues or the automatic time-outs. Because OWA is designed to be accessed from any computer with a Web browser and an Internet connection, it is inherently less secure than its client-based counterpart, Microsoft Office Outlook. That’s why OWA prompts you to provide your location information before you log on. By assigning different time outs based on user location, OWA helps reduce the possibility of unauthorized personnel accessing your mail.
More info: MOS Blog