ERROR This error is the most common one experienced by users of Outlook Web Access, also known as OWA. You will typically see the error after selecting your CAC certificate and before accessing your mailbox. If you see this screen you will have to close every instance of your browser window and relaunch before you can successfully log in. This error occurs when you do not select the correct email certificate.
The typical error process
Browse to https://web.mail.mil and click OK.
Select your EMAIL certificate and click OK*
Click your pod link
Select your EMAIL certificate again and click OK*
*-Because the error is most often tied to the certificate you select there are two points in the process where you may encounter this error screen.
Solution! If you encounter this error screen at any point you must close every internet browser window you have open and then start a new browser session. If you receive the above error without the system asking you to select a certificate it is because there is still a browser process open. A reboot may be necessary if closing the browser window does not correct this issue.
TIP: Tom in our comments suggests you use the Compatibility View mode for the mail .mil site.