e·pis·to·lar·y 
(ĭ-pĭstə-lĕr′ē)
(ĭ-pĭstə-lĕr′ē)adj.
1. Of or associated with letters or the writing of letters.
2. Being in the form of a letter: epistolary exchanges.
3. Carried on by or composed of letters: an epistolary friendship.
[From Latin epistolāris, from epistola, epistle; see EPISTLE.]