File application in trial court for de freezing of account. Supritendant has no right to freeze the account, however, department has every right to recover the lost amount by fraud. . 

1. No authority has any right to freeze any account without the express orders of the court. Hence, this act of Superintendent of Post is not only blatantly illegal but also ultra vires the constitution.
2. As to your legal remedies, it depends whether you are name in the FIR  or not. 
3. If you are named in the FIR as co-accused, you have to appear in the next hearing of the case going on and inform the court of the blatant illegality committed against you of freezing your bank account without court orders to do so and seek exemplary and punitive compensation. 
4. If you aren't the co-accused, you have to sue the Postal department and Union of India. Whether you have to go for Writ Petition or civil suit before district court can best be adviced to you after perusal of the documents, the letter with the bank and so on. So you must visit me for further exhaustive consultation. 
5. For both 3 & 4, you do require competent legal services to litigate. I extend my services. 
6. I'm based in Mumbai & NaviMumbai just as you are, so it shouldn't be much of an issue for you to visit me for exhaustive consultation stated in 4.
7. ९८२०८९७८८४ (nine eight two zero eight nine seven eight eight four) 

Has your father been implicated as accused in charge sheet kindly clarify 

if he has not been made accused your father and other family members should file writ petition in HC to unfreeze tneur bank accounts 

Is your father charged sheeted as an accused ?

Any police officer can seize the account but it should be Investigating officer. When CBI is investing the case than SP cannot direct freeze. 

If there is any violation in following the procedures of the Cr.P.C., the freezing of the bank accounts cannot be legally sustained.

First application in court where trial is going, if refused than approach high court. 

Generally on executing the bond, court orders unfreezing. 

The authority to request a freeze on bank accounts is reserved for regulatory bodies like the Reserve Bank of India, Securities and Exchange Board of India, income-tax officials, or courts. A bank has the right to halt operations or freeze a bank account in certain situations.

Banks often follow account holders’ instructions when making payments. The functioning of a bank account, however, may occasionally be suspended or frozen if the account holder’s instructions are refused.

In reality, it will take a lot of time and effort to get the account in order once it has been legitimately frozen. Now, in order to define an unlawful transaction, you must consult the applicable rules or the bank.

You may issue a legal notice to the bank to unfreeze the bank account failing which you can approach court with a suit or a writ petition against the bank for this act or even can approach CBI to issue instructions to bank to make the account in order if the case has been disposed or if your father has not been implicated in the ongoing criminal case

You need to approach the court where the jurisdiction lies for defreezing the account 

Dear Client,

The authority to seize a bank account lies with the Investigating Officer, particularly in cases involving the Central Bureau of Investigation (CBI). The freezing of accounts should adhere strictly to the procedures outlined in the Criminal Procedure Code (Cr.P.C.); any deviation from these procedures would render the action legally unsustainable.

If a request to freeze an account is initially denied by the court where the trial is ongoing, the next course of action would be to approach the High Court for further consideration. Typically, upon the successful execution of a bond, the court may order the unfreezing of the account.

- Legally , the Superintendent having no right to freeze the account without getting direction of the CBI or any court of law, 

- Further, no action can be taken against the family member of an accused under any provision of law , and hence the freezing of the account of the family member is against the law.

- Since the charge sheet has already filed before the court, then you can move an application before the court for passing an order of de-freezing of the account . 

You should approach the authority which ordered the bank to seal the account and if they do not help you then you may have to approach Court of law to seek directions to the concerned authority.