Debt collection notification by email?

I just checked my spam mail and found an email entitled “Personal and Confidential for [my name] regarding [property management company]” from Hunter Warfield, Inc. It contains my name, the name of my old rental property management company, an account number and PIN, and a PDF showing a balance that I “owe.” The debt absolutely exists. The problem is that the debt is absolutely not mine.

I moved out several months before this debt was accrued. I rented with a squatting ex-cousin/roommate who refused to leave despite me setting him up to do so properly and without having to accumulate debt. I paid an extra month before I left despite knowing I would not be living there (though this was mostly to protect my record from showing his delinquency if I didn’t find an apartment fast enough, which didn’t end up being the case).

When I left, I gave proper 30 day notice, it’s in writing in multiple ways that I gave sufficient notice (initiated through the resident portal and also through email), I returned every single key, removed all my personal belongings, and updated my address. I have not resided there since September of last year. The lease was month to month, so logically I feel like there’s no reason I should even be held responsible for my ex-roommate squatting there from November to January until eviction. But I guess legally that isn’t the case.

At no point did I ever receive any legal correspondence about this; no court summons and definitely no certified mail from Hunter Warfield. I’ve never been in a situation like this before, that apartment I shared with him was the first time I ever rented, and I’m just wondering what my options and next steps are from here?