If you're unable to log in, especially during the evening, or late afternoon, it is most likely due to IT services updating or fixing their Dbs (unannounced).
This issue may also occur with Vista E-learning during the evening. For E-learning, after you've gone past the first 2 pages of logging in, and arriving in the final "Log In" with Username & Password page, you'll notice the different interface, with the name: UNSW Domain E-learning. If you type in your username & password, it will state incorrect.
During these periods, only IT services staff are able to log into Unimail and/or Vista E-learning. These usually last half an hour to an hour.