I would not ignore it... state the facts of your case clearly with any supporting data. Mail it to them, then forget it.
The IRS actually operates on a form of profit motive. They pursue what looks like easy money. If they can shake loose some extra money with a computer generated form letter they will. If you don't owe them, tell them so and then forget it.
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Decades ago I tangled with the IRS over a limited partnership investment in a R&D tax shelter. These were popular back in the 70s and a cleverly structured tax shelter investment could generate tax savings of several times the initial investment. My technician job back in the early 70s was working for one such R&D tax shelter business. I twisted the arm of the general partner(s) to allow me to invest. My only $3k investment was the smallest partner in the entire group, but thanks to the tax benefit it didn't cost me anything, actually netted me a profit over a few years of returns.
Long story short, the government rescinded the overly generous tax shelter law to prevent abuses like that. The IRS put a lien on my tax return (actually all of the partners) asking me to voluntarily extend the statute of limitations for another year because their legal review was not finished. I agreed to extend, but only for my specific liability associated with that partnership investment. One year later they asked me for another one year extension... this time my only liability was for that one tax shelter investment, at most a couple $K, I told them to buzz off, and they did. I was too little for them to come after.
So be little to the IRS.
JR