I had a close relative who was severely paranoid schizophrenic l, and she was known to write letters to the President (of the US) and would claim to have gotten personal replies from him. This relative would also write letters to local law enforcement about imagined infractions by the neighbors, asking for investigations, arrests, and specific numbers of years of incarceration for the (alleged) ne'er-do-wells.
This letter is very much in that style.
I'm sure the Office of the POTUS as well as the police have experience with letters like this, as delusional and/or grossly misinformed people are unfortunately pretty common.
You know you're doing things right, so if the IRS develops an interest because of her letter, then fine, you'll interact with the IRS about it...but you need not reply to this person, because if they are paranoid schizophrenic or uninformed, they won't care about your response. They care about someone paying attention to them, or to acknowledge their "world."
A possible response, if you feel a need to do so, would be something short and sweet, would be:
"Thank you for taking the time to contact me about this matter. I assure you that I will give your email/letter all due consideration.
With appreciation,
(signed)"
Keep it available for a quick copy and pasted (or do one of those fancy schmancy "macro" things I've heard about) and voilá!, a short and sweet "now bugger off" reply.