Taken from: Poetry.org

Epistolary poems are, quite literally, poems that read as letters. As poems of direct address, they can be intimate and colloquial or formal and measured. The subject matter can range from philosophical investigation to a declaration of love to a list of errands, and epistles can take any form, from heroic couplets to free verse.

Elizabeth Bishop’s “Letter to N.Y.," for example, uses rhyming quatrains:

In your next letter I wish you’d say
where you are going and what you are doing;
how are the plays, and after the plays
what other pleasures you’re pursuing:

taking cabs in the middle of the night,
driving as if to save your soul
where the road goes round and round the park
and the meter glares like a moral owl

The rigid rhyme scheme and structure of the poem belie its mournful, almost obsessive nature. Bishop employs the direct address to express her isolation and longing while maintaining formal distance.

This is counter to epistles that assume the more recognizable conventions of a letter, complete with a traditional opening address, such as Langston Hughes’s “Letter," which begins: “Dear Mama / Time I pay rent and get my food / and laundry I don’t have much left / but here is five dollars for you.” The simple intimacy of the epistolary form gives the poem a familiar ease.

The appeal of epistolary poems is in their freedom. The audience can be internal or external. The poet may be speaking to an unnamed recipient or to the world at large, to bodiless entities or abstract concepts.
As usual, feel free to post your finished pieces in the main forum for feedback.