Enforcement Rules of the Employment Insurance Act Article 21

When an applicant submits his employment and non-return cards, pursuant to Article 27, Paragraph 1 regulations of the Law, or when an insured person claiming unemployment benefit notifies the Public Employment Service Institution of his reemployment, pursuant to Article 22 regulations of the Law, this should be submitted in person or sent via registered mail; for persons using registered mail, the date of posting shown on the postmark will be taken as the date of application.
中文原文 就業保險法施行細則第21條
申請人依本法第二十七條第一項規定檢送就業與否回覆卡或領取失業給付之被保險人依本法第三十二條規定通知公立就業服務機構再就業時,得以自行送達或掛號郵寄方式辦理;其以掛號郵寄方式辦理者,以交郵當日之郵戳為準。

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