@Jason: Since you're seeing ALL users logged out simultaneously this would indicate the SmarterMail IIS site or application pool might be restarting/timing out. Can I have you check to confirm its set up not to use Start Mode: Always Running, has no periodic recycle time configured, and is set up to restart only once nightly? These settings are all found within the Advanced Settings for the SmarterMail application pool in IIS.
Kyle Kerst
Lead Internal Network/System Administrator
SmarterTools Inc.
smartertools.com