Tex. Elec. Code § 251.007
When this title requires a 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 or common or contract carrier is timely, except as otherwise provided by this title, if:
Amended by Acts 1987, 70th Leg., ch. 899, Sec. 1, eff. Sept. 1, 1987.