If you have no reason to know that your spouse is cheating on their taxes, it makes sense that you shouldn’t be responsible for the consequences of your spouse’s actions. The IRS agrees, but until recently it rigidly enforced a two-year limit on when a claim for innocent spouse relief could be filed. Because an innocent spouse may not even know that their spouse is in a dispute with the IRS for several years (e.g. they don’t see the mail) the limitation period didn’t make sense.
The IRS recently announced it is no longer enforcing the two-year limitation for all equitable relief cases, whether recently filed or not. In fact, the IRS announced that where litigation is final and the courts have ruled against an innocent spouse because of the limitation period it will suspend collection activity.
This rule only applies to requests for equitable relief tax liability. If you don’t qualify for regular innocent spouse relief (for which the 2-year statute of limitation still applies!) you may still qualify for equitable relief. Equitable relief is basically throwing yourself at the mercy of the IRS. It stems from ancient English common law where a person found guilty of a crime or civil liablity could ask for mercy, i.e. equity, from the King.
For more information, read IRS Publication 971, Innocent Spouse Relief