Unfair termination from company after my intended resignation

Hello , I am working for an MNC IT firm from last 7.9 years and I resigned on 10th of July when i was on deputation in US. My HR contacted me in US number and tried convincing me to return to India and serve 3 months notice period . I told them that i am willing to pay whatever is the settlement amount in lieu of the notice but they kept stating that resignation while on deputation is invalid. I have never signed or seen any such clause that states that resignation while on deputation is invalid, moreover i was deputed on an H1B Visa and i have seen many of my colleagues resigning on H1B in my own project. The HR's final mail on 16th stated that i need to come back to India otherwise set protocols will be followed. I wrote many mails stating that i am willing to pay the settlement amount but got no response after 16th July during which i had submitted all company material like laptop and token etc. Even my official Email ID was blocked thereby giving me no official channel to communicate. On 20th of July the HR posted a registered post to my permanent address in India stating that my resignation will be accepted if i respond within 7 days to the letter either by serving 3 months notice period or by paying 3 months basic pay. This letter reached my permanent address well past the required response date i.e. it reached on 29th July thereby giving me no chance to respond within time.I have IndiaPost registry proof of this from website. Also, even though the HR department and the executive concerned were well aware that i am in US and were aware of my contact details they did not even tried informing me by any other faster modern means. I responded on 1st August just as i received the information confirming my willingness to pay 3 months notice period amount .But later I received another registry letter on 3rd August at my permanent address which was dated 27th July ( intended response date ) stating that my resignation is not acceptable and that they have terminated me from the company as i neglected or failed to reply before 27th July. The HR told that i will not get any releiving letter and will only be eligible for the service letter. I even came back from US leaving my new job and contacted them stating that it is unfair and that i have come back to India just to see that the matter be resolved amicably as this is my first company and i have given 8 years and it will be detrimental towards my career if they terminate me on such unfair grounds but got no encouraging response from them. Please suggest what should i do as i am really very stressed. I have been writing mails to HR executive stating the unfair treatment meted out to me with the heads of departments in CC but only receiving same reply that i should have returned back when i was asked to and that my termination is final. Can i take any legal recourse ? Thanks.