It's... It's okay to use these in letter writing, but not when you're writing to a client, to your boss, in the workplace, on the IELTS if it says, you know: "A formal letter", don't use these words.
其實這用在書信寫作是沒問題的,但前提是你並不是寫給某個客戶,老闆,或在工作場合當中使用,或者是雅思考試中, 你也知道,如果題目說:"一封正式書信",就別用這些字。