I had work for this employer for 6 yrs in the state of Texas and had signed a non-compete contract.

I had work for this employer for 6 yrs in the state of Texas and had signed a non-compete contract. After the 6yrs of hardship with this company basically they had work me like a dog and I was under paid. I had decided to resign and go on my own business venture. In doing so, I had asked them to release me of all contracts every signed with them. Of course the critical one was the non-compete contract that I was worrying about. The employer had agreed which they signed a release of “ALL” contracts and also release of “ALL” claims against me (signed by both me and my ex-employer and I have a copy of the letter). After my departure, due to the poor economy I decided not to risk my own business venture that’s when a competitor approached me and offered me a job. I told them about the non-compete contract I had with ex-employer but I do have a release of “all” contract and claims signed by my ex-employer. Their attorney reviews the contract and told them that I was clear to work for them. After coming on board I received a letter from my ex-employer accusing me of soliciting their existing customers and warned me about my ongoing legal obligation with ex-employee. They even indicated that I had violated the Texas common law trade secret protections. To my understanding, if I am released of “ALL” contracts then how can they proceed the law sue?
They did not only send the attorney letter to my home address but also send it to my current employer. My current employer told me to blow it off because they have nothing against me. My question is should I blow it off? And what are my legal rights? Can I be sued even they release me of all contracts and claims against me? Or can I sue my ex-employer for harassment at my current employer?