Fake ISP

As registration requests become more sophisticated, more tools become needed to combat online sites from getting access to email addresses. While plenty of sites will go so far as to not even require email addresses (with the warning that not providing it means no password recovery services are available), some go the other way and refuse to accept email addresses from free email providers, and checking services that provide such accounts publicly to prevent them from being useful.

So, the next step would be to produce a web site that pretends to be a DSL provider. Attempts to register phone lines would indicate that the service was not available in the requestor's area, attempts to login would normally fail. However, users with the correct username and password (spread by word of mouth) would be able to login in and produce email accounts.

I suspect this strategy would fail quickly if the site became too well known, but it would make a nice defense against registration services which, rather than making a list of which domains belong to free email providers, keep a list of which ones don't, and manually add new ones.