IMT have identified a Microsoft update which may be the cause of users being unable to access webmail.
To work around this you can put the browser into compatibility mode. To do this you will need to be on the webmail login screen. Right click the broken page icon next to the address bar and select compatibility mode. Alternatively you can click ALT (to get to the menu) and then select Tools and Compatibility View.
A further update will be issued at 15:00
IMT systems are continuing to investigate this issue and a web page will be made available shortly to describe the work around in more detail.