Why are certain emails banned?

Certain email providers are blocked due to excessive spammer use. We don't block individual email addresses. We block at the mail provider level

gmail.com, hotmail.com, ymail.com, ymail.org, ymail.net and cmail.com have all been blocked from being entered in the automated registration, and from users changing their address themselves to these addresses. These are because those sites do not adequately check for humans when creating new email accounts. Blocking these reduced the spammer registrations by (in total) over 200 per day.

Blocking them saves the admins over 2 hours of work per day and allows the automated registration system to remain in use.

Users with existing accounts may Private Message Aramis, Cryton, or McPerth, listing the current registered email address, the new one they want it sent to, and a nicely worded request, and we'll change it over, even to one of the blocked email providers.

New users are asked to use a non-blocked email address for registration, then to PM one of the above mentioned admins for the change.