怎樣用英文回覆客人恭喜我升職的信, 急!急!急!

客人恭喜你, right?

你可以咁樣回覆 ----------->

Dear XXX(客人嘅姓名) / To whom it may concern(如果你唔知道對方嘅姓名),

I would like to take this opportunity to express my heartfelt thanks to you and I do appreciate your congratulation.

如果你同你個客人好熟 ﹐你可以----->
[講下你升職嘅心情.......blah blah blah]
OR
[如果你同你個客人唔係好熟 , 你就唔需要講任何野]

Once again, thank you so much!!!

Very sincerely,
XXX <-----your name

其實, 寫 appreciate letter 唔需要寫得咁 detail, 簡短直接就足夠~ =)