How does 雲 + 朵 compound to mean 'your letter'?
In Chinese, 「朵」 is a measure word for both flowers and clouds; this measure word does not have an equivalent in English. 「朵」 is (for example) analogous to cup in cup of coffee; a phrase like 「五朵雲」 is translated idiomatically to English as five clouds, but broken down as


  five 「朵」 of clouds


just like


  five cups of coffee




「朵雲」 refers to the personal signature of a Tang Dynasty duke named 韋陟, son of 韋安石. It is said that when the duke signed off his letters, he wrote his personal name, 「陟」, with an elegant flourish that made it appear as if it was drawn as five clouds.

This signature was admired so much by the duke's contemporaries, that 「朵雲」 became a fancy dub, or an elevated or honorary way of saying, your letter; that is,


  your letter is so elegant and beautiful that it's as if it was signed off by 韋陟 himself, complete with the flourishing five-cloud signature.




See the 《漢語大詞典》 entry on 「朵雲」 and 「郇公五雲體」.