Outlook is having trouble connecting to Exchange.  When you test webmail, the login interface comes up, but after submitting the form the following error is displayed:

Outlook Web Access could not connect to Microsoft Exchange. If the problem continues, contact technical support for your organization.

Exception
Exception type: Microsoft.Exchange.Data.Storage.ConnectionFailedTransientException
Exception message: Cannot open mailbox /o=[org]/ou=[org_unit]/cn=Recipients/cn=[common_name].

…

Inner Exception
Exception type: Microsoft.Mapi.MapiExceptionLogonFailed
Exception message: MapiExceptionLogonFailed: Unable to open message store.

I checked Exchange and the services were running fine.  Windows popped up an alert right after to see drive space was low.  Only 10 MB was available.  Freeing up space brought it back online immediately.

If we had Nagios or similar setup on our corporate side to monitor our IT infrastructure this could have been caught well in advanced.  It’s certainly come in handle at our colo.