One who writes epistles or letters; a letter writer or correspondent.
From epistle plus the agent suffix -er, which creates a noun meaning 'one who does X.' Similar to baker from bake or teacher from teach. This is a direct English formation.
An epistoler is basically anyone who writes letters seriously, but the word has mostly died out—we'd say 'correspondent' today, which shows how our vocabulary evolves as our communication methods change.
Complete word intelligence in one call. Free tier — 50 lookups/day.