26 CFR § 301.6303-1 - Notice and demand for tax.
(a) General rule. Where it is not otherwise provided by the Code, the district director or the director of the regional service center shall, after the making of an assessment of a tax pursuant to section 6203, give notice to each person liable for the unpaid tax, stating the amount and demanding payment thereof. Such notice shall be given as soon as possible and within 60 days. However, the failure to give notice within 60 days does not invalidate the notice. Such notice shall be left at the dwelling or usual place of business of such person, or shall be sent by mail to such person's last known address. For further guidance regarding the definition of last known address, see § 301.6212-2.
(b) Assessment prior to last date for payment. If any tax is assessed prior to the last date prescribed for payment of such tax, demand that such tax be paid will not be made before such last date, except where it is believed collection would be jeopardized by delay.