Postal Act Article 39
Intentional refusal to return mail items that were received by mistake shall be subject to a fine of NT$2,000 to NT$10,000.
中文原文 郵政法第39條
誤收他人之郵件，故意不返還者，處新臺幣二千元以上一萬元以下罰鍰。
English translation for reference only; the Chinese text prevails. Not legal advice.