By default the Outlook Web Access (OWA) will automatically time out for the security purposes. This feature has been designed to restrict unauthorized access to any mailbox when the user is using a public or shared computer. You can select this option before you logon to your mailbox.
Though this feature is good for security reasons it may be annoying for many users who use OWA regularly and they may not want to enter the password several times after the time out. This can settled down with a simple registry tweak on the CAS server that runs your Internet facing OWA site. This can be done by following registry modification:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeOWA
Name: PublicTimeout
Type: DWORD
Value: {value in minutes} (This value is 15 minutes by default)
The above suggestion applies only when the user selects the Public Computer option from the OWA logon screen. For the user who select the Private Computer from the logon screen you might want to modify:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeOWA
Name: PrivateTimeout
Type: DWORD
Value: {value in minutes} (This value is 8 hours by default)
If you don’t see the DWORD values named, PublicTimeout and PrivateTimeout then you have create then manually.