Some small group of customers still have access to custom spam filters. Custom spam filters allow you to select filtering by blacklist.
Why is this feature no longer available? While it sounds great in theory, in practice, if a blacklist's effectiveness changes, or we want to add a new blacklist, or a blacklist goes away, your effective spam filtering rate has a tendency to drop. If your effective spam filtering rate drops too low, if your spam can become a large enough percentage of delivered mail, your email provider may block all Pobox mail. This is not an imaginary possibility -- it has happened before, and is why spam filtering is required on all accounts.
Effective March 27, 2015, all custom users will, at a minimum, have their mail filtered by our current most effective, accurate filter. Messages caught by that filter will be discarded. You can select a more aggressive action (bounce that mail), but you cannot select a less aggressive one. This change is required to ensure the deliverability of all customers' mail.