General rules:
1) Spousal support is considered to be alimony Unless the agreement specifically states that it is Not alimony.
2) Alimony is a deduction for the payer, and taxable income to the person being paid.
3) Income is taxable in the year which it is transferred, regardless of what year(s) it was for.
Thus, per your clarification (and pending further info), husband would owe tax on the entire lump sum when received, and other spouse would have a corresponding deduction for the same amount.
