Highland Capital Debt Collection Complaint

Highland Capital Debt collection department,

Payday loan Taking/threatening an illegal action Threatened to sue on too old debt California

I was contacted by this company who is threatening to sue me. So i entered into an agreement to repay the debt. I was sent an email of a letter after i asked that a physical copy be mailed to me. i then noticed small typos and no street address only a po box . However i was unsure of the laws, and felt a little uneasy so I checked my credit report and I did not see the debt they are attempting to collect so I assumed it had been charged off or was too old to be collected. After trying to research the company I am now afraid that they may not be legitimate and maybe be trying to collect money from me for fraudulent purposes.