Where a deadline for submission is specified in this Part for submissions by owner or tenant to the DHCR, such submission must be filed in person or by mail, or as otherwise provided in an operational bulletin issued pursuant to section 2527.11 of this Title, by such deadline. If the submission is filed by mail, it must be postmarked no later than such deadline. If the prepaid postage on the envelope in which the submission is mailed is by private postage meter, and the envelope does not have an official United States Postal Service postmark, then the submission will not be considered timely filed unless received by such deadline, or other adequate proof that the submission was mailed by the date specified, such as an official Postal Service receipt or certificate of mailing, is submitted.