Any correspondence or communication written in the form of a letter or series of letters is said to be epistolary.

They've gone out of fashion now, but in the eighteenth and nineteenth centuries the "epistolary novel," written in the form of fictional letters to and from various corespondents, was all the rage. One of the most famous such novels (or at least part epistolary) is Bram Stoker's "Dracula." Epistolary is related to the word epistle, a fancy term for "letter."

Definitions of epistolary
  1. adjective
    written in the form of or carried on by letters or correspondence
    synonyms: epistolatory
    used of spoken and written language
Word Family