It appears that you have submitted resignation by email. It is still not made clear by you that you have submitted notice of resignation with effective date of resignation mentioned in it or you have submitted resignation?
You may submit notice of resignation with effective date of resignation clearly mentioned in it as a subsequent communication to your email, preferably by letter, under proper acknowledgment by redg/speed post, addressed to your appointing authority, and enclose copy of your email, and obtain acknowledgment. Submit reminders after a few days and finally on the effective date of resignation submit formal resignation, under proper acknowledgment.
If you have settled everything from your end e.g. completed all tasks on hand, handed over the charge and company property (under proper acknowledgment preferably with comments that nothing is pending at your end), submitted proper notice of resignation and served notice period (mark attendance and collect attendance record), and any other item which need to settle as per company policy, then nothing is pending to be settled by you.
Company should not sit on relieving letter and issue it. Relieving letter is issued post all settlements.
You should request the company to supply you acknowledgment of notice, acceptance of resignation, settlement of your a/c and payment of your dues, FNF statement, work experience/service certificate, form 16, PF accumulation report and withdrawal/transfer forms, NDC etc. Company can not block these docs. It shall be better if you obtain these with your reasoning, negotiation, persuasion, persistence skills.
Acknowledgment of notice of resignation, Proof of submission of resignation, acceptance, FNF statement, work experience/service certificate, acknowledgment of handed over the charge and company property should suffice. Your new employer at the most may ask you to sign an affidavit that you are not employed elsewhere.
However it shall be appropriate if you communicate in writing to new employer that you are in a position to supply only copy of notice of resignation, resignation, receipt of regd/speed post, POD, only, and can submit acceptance, work experience certificate/reliving certificate only upon its receipt from previous employer.
If company does not settle you can approach the o/o Labor commissioner.