Tex. Election Code § 1.008: TIMELINESS OF ACTION BY MAIL.

When this code requires an application, notice, report, or other document or paper to be delivered, submitted, or filed within a specified period or before a specified deadline, a delivery, submission, or filing by first-class United States mail is timely, except as otherwise provided by this code, if:

(1) it is properly addressed with postage prepaid; and

(2) it bears a post office cancellation mark indicating a time within the period or before the deadline, or if the person required to take the action furnishes satisfactory proof that it was deposited in the mail within the period or before the deadline.

Acts 1985, 69th Leg., ch. 211, Sec. 1, eff. Jan. 1, 1986.