The accounts have frequently posted photos, videos and text attempting to recruit new members and celebrate violence. And they're not all small-time Twitter users, either. Some accounts had "tens of thousands" of followers. While it's doubtful that many of those followers would be receptive to ISIS, that still represents a large audience.

Twitter suspended most of the accounts TechCrunch saw. In a statement, a spokesperson said that recycling email addresses like this is "not a new issue" and that the company was "working to identify solutions" that could protect Twitter accounts.

It's true that things have gotten better (Twitter now verifies emails during sign-up), and some of the blame sits with email services that allow addresses to expire. However, this still suggests that Twitter has room to improve its anti-terrorist efforts, such as requiring email confirmation for older accounts. That could prevent ISIS and other groups from hunting down abandoned accounts when their own handles face bans.