Maryland Code, Election Law § 13-322

(1) regardless of when it is received, the United States Postal Service has affixed a mark on the envelope or on a receipt verifying that the campaign finance report was mailed on or before the filing deadline; or

(2) it is received by the State Board within 3 days after the filing deadline and a private postal meter postmark or a receipt by a private carrier verifies that the campaign finance report was mailed or delivered to the private carrier on or before the filing deadline.