Currently I use the "banned emails" list to disallow all Microsoft emails, because they completely block our emails (doesn't even go to the spam inbox) so users will never get their confirmation email. This also means that many of our users who previously used Microsoft emails can no longer get site mailings. I figure it'd be a neat feature to have an option on notices along the lines of "User Has Banned Email" so a message urging them to change it could be displayed. This feature can already be accomplished by entering the emails under "User's email address contains", although I think it'd be neater to have it managed by the banned email list instead. A "Reason" option with each banned email could avoid confusion. New users tend to believe that because their email is disallowed, that it is taken.