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:
"The IRS doesn't initiate contact with taxpayers by email, text messages or social media channels to request personal or financial information"
Additionally, it is *extremely rare* that the IRS would initate contact via phone, and in fact, agents are strongly discouraged from doing so. A phone call where someone claims to represent the IRS and states that you will be arrested unless you make an immediate payment is almost certainly fraudulent.