I've paid them, sent proof, they keep harrasing me and threatening me

I have run out of ideas on what to do with these people. I've been getting threats of default listing from them for a while. I respond to mails they simply ignore me. Latest one is them confirming a listing. I am a land lord to one property which we were unfortunate for the estate to have chosen them for rates. Tenant was paying them directly. When they got replaced, probably due to bad service, they decided to come after me. Citing money owed, supposedly by the tenant account. They had access to the deposit but decided not yo hold it. They instead come after me. So I paid it and sent proof of payment. For the life of me I have no idea why they kept on sending me threats for same month after month. I respond but get np confirmation. They are harrasing me and I do not know what to do