Interest in epistolary novels has waned in YA editorial circles, it’s true. But often a format or category isn’t so much “dead” as just in need of a fresh spin to jolt it out of the doldrums. If you’re committed to this format, you’d better be offering something eye-catching in your concept or plot because, as much as I hate to write this, “excellent writing” isn’t enough to break anyone into a stagnant niche. Look for agents who rep projects with your kind of concept, tone, and audience, then emphasize those in your query: “I’ve got this great novel about X”, not “I’ve got this great epistolary novel.” If it’s still a no-go, why not recast your great concept, cast, and plot as a traditional narrative? Loyalty should be to story over format.