your kind letter
A formal, respectful term used to refer to a letter received from someone, emphasizing the writer's kindness or sincerity. Common in polite correspondence.
Thank you very much for your kind letter the other day.
Compound of 懇 (こん, 'kind, courteous') and 書 (しょ, 'writing, letter'). The exact historical derivation is uncertain, but the term is used in formal written Japanese to express gratitude for a received letter.