Initially, you have to write a letter to the concerned authority in the State government officials, explaining the reasons for not joining in time and asking them to extend the date for joining. In this letter, you have to write very clearly the reasons for the delay. The reasons you write should prove that the delay is not due to you but due to the reasons which are beyond your control. Along with the letter you have to attach papers which will prove that what reasons you are telling is real and not managed. If the officials understand the reasons are genuine they may extend the time of joining.
You should ask a letter from the present department that is Postal department to give in writing the various reasons which resulted in a delay in accepting your technical resignation with a lien. A copy of this is to be attached to the above letter.
Till you get the concurrence from the State government you should not get relieved from the present job. If you get relieved from the job and if you are not able to join in the new job, it will be a big problem. If you fulfil all the terms and conditions about resignation, there should not be any reason for nor relieving you on time.
The last resort is to go ahead legally. You have to consult a good lawyer and you have to proceed as per the directions given by the lawyer.