Of, relating to, or having the character of epistles; pertaining to letters or epistle-writing.
From epistolic plus the suffix -al, creating an emphatic or archaic variant (similar to historical from historic, or identical to musical from music). This double-suffix formation is rare.
When you add -al to an adjective already ending in -ic, you get a super-archaic sound—epistolical feels like it belongs in a 17th-century library, and practically speaking, that's where it stays.
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