So somewhere in the US code, you can't pretend to be the IRS, right? Well, these folks in California claiming to be "Federal Tax Authorities" seem to be doing so. It should also be added that seizure of assets is an extremely rare thing these days with a quieter IRS. It is not a common activity except in the rarest of circumstances. Levies? Sure. Liens? Yes.
So, to contrast, here is a real Tax Lien Letter. The consequences of a Tax Lien? If you own real estate and sell your home, the lien will be paid before you do (but after the mortgages).