Harassment at work

Hello! I worked for a top MNC for 8 months. Left on a terrific note like anyone else and got my Full & final settlement with No dues letter and experience certificate within 26 days of leaving in hard copy and via email 3 months after that they sent me a recovery letter stating i owe them a few thousand rupees as per the attachment where as the body of the mail had 10 times the amount. I responded on the email the same day itself seeking a clarification as i had everything from their end on the company letter head in both soft and hard copy. I never heard back from them. 1 month after that i got another mail stating i owed them another very different number much higher than the previous one as well. I again responded to that within 3 days explaining the complete chain of events and as i was hearing from no one in the seperation team i cc'd the person who's signature was on the recovery notice. He replied after 2 days to one of his team members to close loop with me, but no one called and clarified anything. Now after another 1.5 months i get a letter by Registered Post stating the contents of the mail has an annexure explaining the recovery amount, but again it has nothing and they're threatening me with a legal proceeding for not having cleared my dues and not responded to them for 87 days...where as i have all mail trails as evidence to counter propose that including the original mail stating my experience and no dues. I have a great mind of suing the company for mental torture and harassment inspite of having responded to each and every mail. Your thoughts and recommendations would be highly appreciated.