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]{ござ}候、
…の[次第]{しだい}にて候、
…[致]{いた}して候、
…と[思案]{しあん}[仕]{つかまつ}り候、
[之]{これ}あり候、
[有間敷事而]{あるまじきことにて}候、
思い[悩]{なや}み候、
..の[料簡]{りょうけん}にて候、
[暇]{ひま}を取らせ候、
[確]{しか}と[承]{うけたまわ}り候、
[武士]{ぶし}の[一言]{いちごん}、[金丁]{きんちょう}にて[誓約]{せいやく}[申上]{もうしあ}げ候、
[切腹]{せっぷく}申し[渡]{わた}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]{はいけい}、[叔父上]{おじうえ}他御一統様[如何]{いかが}[被遊]{あそばされ}候や。[当方]{とうほう}一同[恙無]{つつがな}く[打越]{うちこし}[罷居]{まかりお}り候[間]{あいだ}[何卒]{なにとぞ}[御放念]{ごほうねん}[被遊]{あそばされ}[度]{たく}[願上]{ねがいあ}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 literature and private letters written before the Showa-Taisho era, and sometimes in the speeches of 能狂言 today.