Got a caution notice from my old employer stating breach of non solicitation contract

I recently joined a new company and after 1 month my old employer is sending me a caution notice with subject line: “Notice in connection with breach of contractual terms of your employment and damaging interest of our company by willfully joining our company’s client.” This is related to the NDA they made me sign during joining. Which stated I can’t join any client of company as there competition. However the company which i have joined i never worked with that client during my tenure at my old company. Is this a valid notice to taken care for? They have mentioned following allegations: As you have breached aforesaid Agreements, Company has carried out basic investigation and found following serious legal issues on your part: Huge loss of opportunities and frustration to Company due to your willful violation of employment agreements which is affecting Company's business. Financial loss to Company in terms of cost of your hiring, on job learning, grooming and providing you opportunity to connect with Company's clients. Other financial losses and damages as mentioned in aforesaid Agreements. Spoiling the environment of the Company due to aforesaid acts which further affects labour turnover ratio in the Company and which in turn affects service delivery level of he Company. Attempts of poaching other employees of the Company. They have asked me to write explanation regarding my violation of said agreement and give credit of amount to Indian Rupees Equals to USD 25,000 within 7 days. If i don’t reply they will presume that i have accepted the claims and will further take legal actions. Can someone help what should be done here?