Wyo. Stat. § 16-4-301 (2026)

(a) Any report, claim, tax return, statement or other
document or any payment required or authorized to be filed or
made to the state of Wyoming or to any political subdivision
thereof, which is:

          (i) Transmitted through the United States mail, shall
be deemed filed or made and received by the state or political
subdivisions on the date shown by the post office cancellation
mark stamped upon the envelope or other appropriate wrapper
containing it;

          (ii) Mailed but not received by the state or
political subdivisions or where received and the cancellation
mark is illegible, erroneous or omitted, shall be deemed filed
or made and received on the date it was mailed if the sender
establishes by competent evidence that the report, claim, tax
return, statement or other document or payment was deposited in
the United States mail on or before the date for filing or
paying.
     (b) In cases of nonreceipt of any report, tax return,
statement or other document or payment required by law to be
filed or made, the sender shall file with the state or political
subdivision a duplicate within thirty (30) days after written
notification is given to the sender that the state or political
subdivision did not receive the report, tax return, statement or
other document or payment or paragraph (a)(ii) of this section
does not apply.