Your firm is using a product call "NavGuard" from Messageware. It is a security add on to Outlook Web Access (OWA) that ensures that you don't go off to another website without first closing your OWA session. It is designed to protect your corp mail so that OWA sessions are not inadvertently left active on public machines for the next guy on to fiddle with.
The software can be configured to determine if you are using a secure pc or a public one. See your administrator if device recognition is a factor in what you are doing.
If you want to learn more about this product (and your company is likely using some other Messageware OWA security and enhancement products in conjunction with NavGuard) then check them out at
www.Messageware.com