An epistle is a poem written as a letter to a specific person or group. It often speaks directly to the addressee and may be thoughtful, personal, instructional, or satirical. Unlike a casual letter, an epistle is shaped as a poem, so it uses line breaks, imagery, rhythm, and other poetic effects to develop its message.
Example
Dear Friend,
I sent my worries out with yesterday's rain,
and found the garden clearer when I looked again.
Take this small lesson: even bare branches
keep their strength through winter.