traffic ticket after 6 years

Nothing, in Italy the statute of limitations is five years after the last registered letter asking you to pay. Did you receive any registered letter from Italy? Or, at least, did you notice any strange charge on your credit card a few months after your trip? That would be the rental company's fee; a fee you must pay because they had to tell the cops who broke the law while driving one of their cars.

Some US collection agency must have bought a bunch of unpaid fines without bothering to check which ones are legal and which not.

PS
Did you drive in Italy in 2014? Did you know what a ZTL is? Because you know... the debt collector may be wrong, but maybe you actually deserved to be fined in 2014!

No letters at all. I was not aware of this until a few days ago. I pay my debts as I make them but this has add on after add on and after six years feel no real attempt to notify me ever happened. One also worries about scams nowadays but we were in Italy and could well have gotten an unknown speeding ticket.

We received 2 tickets in Italy in 2018. We received an email from the rental agency about 9 months later with copies of the two tickets attached. Luckily my husband reads Italian and just paid them through our credit card to Italy directly (Venice) our rental agency was autoeurope in Croatia. The point I am trying to make is can you contact the rental agency and ask them if they ever received the tickets.

Identical question with the same answers ---- See ---
https://community.ricksteves.com/travel-forum/italy/traffic-tickets-from-five-years-ago-how-can-i-confirm-these