You're not being sued. That's a cease-and-desist letter. You're violating Twitter's trademark by selling a product with "Twitter" in the name. The legal precedents for this are ironclad. You can't do it.
You're violating Twitters terms of service by offering a program that auto-follows thousands of people at a time. Did you really think you were going to get away with that? It doesn't matter if you think it's fair. It doesn't matter who else is doing it. It doesn't even matter if you think you're right: you're not. Stop infringing Twitter's trademark and stop violating their terms of service and they'll leave you alone. It's really not rocket science.