Bad Reference/legal advice?

The fact is you received a job offer which means you ped the background / criminal/ education and reference check. In your profession, I can't imagine a potential employer extending a job offer without a thorough reference check. So, with that said, keep your composure, and advise your new employer of this letter that he will be receiving, explain that the management wrote it out of anger because you resigned. . When explaining, make sure you do so in a professional, calm demenour, don't say anything negative or go on and on about how unfair your ex employer was. Your new employer will obviously recognize that this is totally out of line and completely unprofessional and most likely will ignore the contents of that letter. Good luck!