I don't know anything about it, but here's some information from a law firm in NY that does prenups:
They discuss some of the grounds on which they think prenups can be challenged in NY. If your ex did not disclose all his assets to you before you signed the prenup, or if you did not have your own separate lawyer advising you about the prenup before you signed it, maybe a NY court might invalidate the prenup, according to them.
Just noticed you wrote "postnup" not prenup. I'd guess the same issues would probably apply.