It seems that outlook.com addresses are a favorite among spammers. I have installed a plugin called “Ban Hammer” which prevents admin specified email addresses and email domains from being able to signup.
If you use an outlook.com email address and really want to signup, please find another email host. I won’t be un-banning this domain any time soon. Sorry for the inconvenience.
Maybe more later…
Edit: I’ve added hotmail.com to list as well.