The Collaborative International Dictionary
Epistolical
Epistolic \Ep`is*tol"ic\, Epistolical \Ep`is*tol"ic*al\, a. [L. epistolicus, Gr. ?.] Pertaining to letters or epistles; in the form or style of letters; epistolary.
Wiktionary
epistolical
a. (alternative form of epistolic English)