If any letter or correspondence or legal notice sent by RPAD or speed post is refused by a recepient, the postal department will endorse on the cover of the said correspondence as "refused" and will be returned to the sender. Since the notice is refused, it is deemed as a valid service and the sender can proceed with suit before the court of law for recovery of money spent by them or loss suffered by them due to your act of leaving the job.
The remedy can be to issue a suitable reply notice, defend the case and make a counter claim, if any or better to negotiate with company about the compensation which you are liable to pay.
Refusing the legal notice is not a correct one
Please document all your correspondence. Have you received the offer of employment? If so, keep the copy of the same with you. Hope you might have put up an application about your salary revision and for that also you might have a copy with duly acknowledged by your HR Department. Then You submit your resignation letter and mention regarding your notice period service or buy back option for notice period.
If you have all these in writing then surely they cannot do anything against you and spoil your future career.