Postal Act Article 18

Stained or damaged postage stamps shall lose all value. The same shall apply to stained or damaged imprinted stamps on postcards, aerogrammes and letter sheets.
中文原文 郵政法第18條
污損之郵票,失其效用。
明信片及特製郵簡上表示價格之花紋有污損時,亦同。

English translation for reference only; the Chinese text prevails. Not legal advice.