Just received a letter in the mail from these peop...

Just received a letter in the mail from these people, after receiving a million other letters from this other company about an account that I paid off in 2011!! If I could find out how, I would sue them. I m sick of the bs. I have absolutely NOTHING in my collections nor on my account regarding whatever account they are sending me a letter about. The other company that said they were a debt collection agency, I asked for them to provide me with proof that I OWED and they provided me with an old statement lol hell I can provide an old statement. It didn t say past due or nothing. I called and they said that they didn t have the rights to submit my account to collections. Of course you don t because I been paid my account off, SO WHY ARE WE EVEN TALKING?!