My clients (a few years ago) got a letter from IRS asking for more information before a refund could be paid.  Since my clients had a balance due on their return, which they paid, they were fairly certain that this was phishing.  I looked at the letter, and it was a pretty good forgery.  

How did IRS want the copies sent?  Was there a fax number, by any chance?  My rule of thumb is that if they send you US mail, you send them US mail.