Hardcode the list of nondisposable email providers instead of making it a config option. Also add a few new providers. This was previously a config option to keep it secret, but there's not much need for secrecy here. A Restricted user's email must be on this list to unrestrict their account. If a user is Restricted and their email is not in this list, then it's assumed to be disposable and can't be used to unrestrict their account even if they verify their email address.
Config
This directory contains configuration files for Danbooru.
To configure your Danbooru instance, copy danbooru_default_config.rb to danbooru_local_config.rb and edit it. See danbooru_default_config.rb for details.
The only file here that end users need to be concerned about is danbooru_default_config.rb. The rest of the files here are internal Rails-related configuration files that end users shouldn't need to edit.