Internet

If you receive anything about a financial topic that is unsolicited, be extremely cautious. This includes email and written notices. 

Over the past few years, scam artists have become more creative and nefarious. There are many that claim to be from a government organization, and are telling you that you absolutely need to click on a link, or open a "PDF", to solve your problem.

At best, the link tries to sell you something, at worst your computer is compromised with a virus.

Here is the quote direct from the IRS: