In general, your tax payments (from withholding or estimated payments or both) must closely match the tax you owe at the end of the year. If you owe a large amount when you file your return, you can be assessed a penalty even if you pay in full when you file. This is an underpayment penalty, for underpaying what you owed over the year. One situation that can cause an underpayment penalty is a lump sum of income late in the year, such as capital gains from sale of property, or a large RMD or other retirement withdrawal, or Roth IRA conversion.
If you had an underpayment penalty in a prior year, it would be reflected on form 2210 attached to your tax return, or you would get a bill from the IRS, or both. You can also check your account transcript. I suppose it is possible that the IRS calculated a penalty for 2024, and this notice/reminder for 2025 was sent before the bill for 2024, but that seems a bit unlikely.
https://www.irs.gov/individuals/get-transcript
Assuming the letter is legitimate, the IRS does not know your current year tax situation. They don't know if you are under-paying or under-withheld this year. And even if you did owe a penalty in a previous year, that does not mean you are under-withheld this year, if your situation has changed. So the letter would just be a reminder to check your finances. We would need to know more about your tax history, such as how much did you owe last year, did you ever get a notice about a penalty, and did your last tax return include a penalty on form 2210, have you had large lump sum income in the end of the year, and so on. If you want to see if your withholding is on track for 2025, use this calculator
https://www.irs.gov/individuals/tax-withholding-estimator
Or, if the letter is a scam, it might be attempting to trick you to make a payment at a fake site. Direct payments to the IRS can only be made at www.irs.gov/payments.
