What did the court decide in Houston v. Lack?
Answered for: US
We thus conclude that the Court of Appeals had jurisdiction over petitioner's appeal because the notice of appeal was filed at the time petitioner delivered it to the prison authorities for forwarding to the court clerk.
Source of the prison mailbox rule: an incarcerated self-represented litigant's notice of appeal counts as filed when handed to prison officials. The sentence above is the court in its own words.
This answer is true in US and nowhere else on this site. Another jurisdiction is another answer, or an honest gap: see what has been read for US.