When your local DRM / IRM credential is corrupted...
Sometimes it happens, that when you try to open a IRMed email , the Outlook will tell you that your IRM credential is corrupted and please contact your administrator, blah blah blah.
on the situation, the fastest way is to just delete all your local DRM / IRM cache located at
c:\users\<your-account-name\appdata\local\microsoft\drm
So that the IRM function can connect to the server again to get the credential.
(remember to close your outlook first and reopen it later when you deleted IRM files...)
FYI