In its original sense, "epistle" refers to one of the 21 letters found in the New Testament.
The apostle Paul, for example, had much to say. What he wrote are considered epistles because they were written in letter form to a particular person or group.
Here's a formal definition of "epistolary" I swiped from my husband's Page-A-Day calendar.
adj.
1. of, relating to, or suitable to a letter
2. contained in or carried on by letters
3. written in the form of a series of letters
Because my grandmother's difficulty hearing, we talk little by phone, carrying on an epistolary friendship instead.
Wow, your husband must be pretty smart if he has a calendar with all them purty words.