Desperately need help on this

I got an offer in a company and I worked there for 2 months in probation period(They have 6 months probation). Now, I got a better offer. So I resigned. But they are denying me the relieving letter. Also the official resignation mail I sent was from office email, to which I don't have access now. And the company where I got the offer now demands some kind of proof with company logo and seal on it. The old company is denying to give any document whatsoever. What should I do? Please help 🙏