Alder Switched Me to ADT

I had a three-year contract with Alder, and I canceled it before the renewal period as stated in the contract. I sent a certified letter to Alder in June to terminate their service, with my contract ending in July. Alder responded with a cancellation letter, which I signed and returned.

Today, I received an email from ADT stating that my account has been transferred to them from Alder. How is this legal, and what should I do?