Colorado Revised Statutes Title 39. Taxation § 39-1-120. Filing--when deemed to have been made

(1)(a) Any report, schedule, claim, tax return, statement, or other document required or authorized under articles 1 to 9 of this title to be filed with or any payment made to the state of Colorado or any political subdivision thereof which is transmitted through the United States mail shall be deemed filed with and received by the public officer or agency to which it was addressed on the date shown by the cancellation mark stamped on the envelope or other wrapper containing the document required to be filed.

(b) Any such document which is mailed, but not received by the public officer or agency to which it was addressed, or is received and the cancellation mark is not legible, or is erroneous or omitted, shall be deemed to have been filed and received on the date it was mailed if the sender establishes by competent evidence that the document was deposited in the United States mail on or before the date due for filing. In such cases of nonreceipt of a document by the public officer or agency to which it was addressed, the sender shall file a duplicate copy thereof within thirty days after written notification is given to the sender by such public officer of the failure to receive such document.