There is a concept of diversion of title at source vs application of income in income tax.
In case alimony is payable when decreed, then this merely being application of income is not tax deductible, since the income is earned by you & subsequently applied, there is no diversion of title prior to acrual of income. Even if as per the decree, an employer pays the sums directly to the estranged spouse claiming alimony under the decree it won't alter this proposition.
In case a vested right is created on the source of income itself then the taxability may be different say a income from business/asset .
As regards taxability in the hands of the recieving party please refer to this case law "Princess Maheshwari Devi of Pratapgarh v. CIT, (1984) 147 ITR 258 (Bom.)".
Kindly look out for the above emboldened sentence on the net for elaboration.