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:
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:
-- 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.