Prenups often contain provisions about how much a partner would pay in alimony, also known as spousal support. The agreements can be thrown out if judges deem them unfair, signed too quickly, or under duress. Now the tax revamp offers another avenue for challenges because courts will likely have to consider how the law has changed since the contracts were created. Starting in 2019, taxpayers will be no longer be able to deduct their alimony payments.