[wdfifteen]

Sounds fishy.
Seven years with no notice, now $4500 in fees?
The IRS charges interest and a penalty on unpaid taxes, not "fees." The IRS notice will itemize your bill - tax + interest + penalty.
Unless they've changed their methods in the past 2 years, the IRS threatens to seize your property after giving notice you owe them money. Ive never heard of them using a collection agency.
The IRS is wayyy behind right now. You have time to investigate this. Write to them, their notices include instructions on how to dispute a bill. Dispute it, say you didn't sign a joint return and never knew about the tax. If this is real you will get a letter in a few weeks saying they need 90 days to consider your dispute (used to be 60 before Covid).
Without having seen the paperwork I suspect this is a scam.

Good luck contacting the IRS. They haven't been answering their phones for civilians for months. You can contact your senators and congressmen to light a fire under them, but that's still slow these days.