POSTMARKS AND POSTAGE METERS
New York State Real Property Tax law section 925 states:
if payment for the amount of any taxes on real property, accompanied by the statement of such taxes, is enclosed in a postpaid wrapper properly addressed to the appropriate collecting officer and is deposited in a post office or official depository under the exclusive care and custody of the United States postal service, such payment shall, upon delivery, be deemed to have been made to such officer on the date of the United States postmark on such wrapper. If the postmark does not appear on such wrapper or the postmark is illegible such payment shall be deemed to have been made on the date of delivery to such collecting officer.
tl;dr: payments received without a US postmark or with a postage meter mark will be marked received on the day they are delivered to our office.
This is of particular importance at or near the due dates. If your envelope arrives after a due date and lacks an USPS postmark, you will be liable for the applicable interest. To avoid this, we highly recommend that you go into a USPS branch and ask that they postmark your envelope while you watch.