Letter 2566, Notice of Proposed Tax Assessment The first notice you’ll receive about a Substitute for Return (SFR) is typically a Letter 2566, Notice of Proposed Tax Assessment. When you receive this notice, the IRS is telling you, “You were supposed to file a tax return but didn’t, and here’s how much we think you owe us.” The proposed assessment is usually higher than it should be because the IRS doesn’t have all the information it needs to file your taxes. They may have a single W-2, but they could be missing your mortgage interest payments, charitable contributions, dependents you can claim, and other deductions and credits you’re entitled to. This notice will give you 30 days to file a return voluntarily. You should do everything you can to file a return during that period to take advantage of all of your credits, exemptions, and deductions.