Yes, payments postmarked on or before the due date are considered an on-time payment.  Keep in mind that placing your payment in the mailbox does not guarantee the envelope will be postmarked that day. The only way to guarantee an official postmark is to go into a post office and ask to have the envelope hand stamped.

IMPORTANT NOTE:  Payments received after the Tax Sale, regardless of postmark, will not be processed and will be returned.