Post employment, Non Disclosure and Covenant against solicitation

Dear Sir, I resigned from a US subsidiary in India after 9.3 yrs. and plan to join their rivals in India. I signed non disclosure during term of employment and thereafter, non compete during term of employment and covenant against solicitation during term of employment and thereafter on the company letter head. Now previous employer has sent me a letter through its legal counsel in US that I should not contact its present customers with whom I have built relationship over the years. the customers are from trenchless industry and the equipment of the previous employer is not a stand alone, the drilling rig is manufactured by the different company and paired with the previous employer equipment (electronic guidance tool) and ready for working. The employment agreement is signed on the company letter head of USA and I was deployed working in India. a) Please let me know the validity of the employment agreement on company letter head or it has to be on non judicial stamp paper duly signed by the Notary/magistrate.