候 is an antiquated predicate meaning “be, is, am, are” as Waley translated. It was commonly used in literature and letters just up until the early part of Showa Era. It is usually used in the statement addressed to your senior, like …にて候、御座候、…の次第にて候、…致して候、…と思案仕り候、之あり候、有間敷事而候、思い悩み候、..の料簡にて候、暇を取らせ候, 確と承り候、武士の一言、金丁にて誓約申上げ候、切腹申し渡し候, and so on.
Actually I’ve written letters in 候文(statements finishing the end of sentence with 候) in my high teens in place of my father who didn’t like to write by himself, in ｓuch a way as, "拝啓、叔父上他御一統様如何被遊候や。当方一同恙無く打越罷居り候間何卒御放念被遊度願上げ候" meaning “Hello my uncle. How well are you and all your family faring off? Our family are all doing well. So please feel at ease."
So I don’t find any difficulty in writing a letter in 候文 even today.
The use of 候 is completely obsolete today, but you may find it being used ubiquitously in the literatures and private letters written before Showa-Taisho era, and sometimes in the speeches of 能狂言 today.