Not Serving Notice period

After resigning from my job I didn't get any info regarding exit formalities or Serving Notice Period. hence I thought I am being relieved from my duties, however when I asked them about my terminal benefits etc they didn't mention anything to me at all. now all of a sudden they are sending me legal notices for not serving the notice period !!! now they wanna charge me to extract money .. my question is they didn't tell me during the course of resignation that to obtain the relieving letter and other things you need to serve exit formalities then only you will be eligible to get out. after resigning, then on then, i handed over my company assets and my systems id , now there's a catch they are saying that we informed you through mail on your company system id mail. whereas i have already handover all of those pieces of stuff during resignation even they received the printed hard copy of resignation letter. ( i mailed them through my company system id and from my personal gmail id and took out prints of it for receiving. they did not said in verbal or written regarding exit formalities after few months they sent me a mail where it is been instructed to pay the compensation asap. now whose fault it is ? should i pay ? or blame them for their incompetence