What can be done when someone submits information to the IRS about you that is false?

We recently received a letter from the IRS stating that we owe thousands of dollars of taxes for income that we never received. My husband was a partner of a LLC. After he dissolved his partnership two years ago, we found out that the other partner claimed to the IRS that we received income that we didn't in the last year of his partnership. Does anyone know what we can do? Isn't is illegal for someone to submit false information to the IRS under your social security number, and to never be informed of it when they do?