Hey...
In my opinion your account might have been hacked. Someone might have got hold of your password and be using your account...
Please take a look at this link...
http://windows.microsoft.com/en-gb/windows/outlook/hacked-account
Hope, this will help you from getting out of your trouble....
Thanks
Hi,
From your description, I would like to verify if your mailbox password is reset by someone unexpectedly. If yes, you can follow the steps below to reset it on ADUC.
Open ADUC -> domain.com -> Users -> right click the problematic user -> Reset Password -> Type the new password
Hope this can be helpful to you.
Best regards,