Capable of being written as or converted into an epistle; suitable for epistolary treatment.
From epistolize plus the suffix -able (capable of being), which creates adjectives meaning 'able to be X'd.' Similar to fashionable, questionable, or readable.
This word opens a fun philosophical question: what makes something 'epistolizable'? Not every story can work as a letter, but give it to the right writer and suddenly it can—like how Bram Stoker told Dracula entirely through letters, journal entries, and documents.
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