Can't fetch config values from config files

I’ve been setting up Bamboo to use for sending emails and the final step I’m taking is setting up the actual mailer to send the email. I’m having trouble getting values from the config files and it seems whatever I put into the config files I can’t read and I don’t think it’s actually a problem with Bamboo but with how I’m doing something.

So I have my mailer

defmodule Recipesite.Emails.Mailer do
  use Bamboo.Mailer, otp_app: :recipesite
end

And I have my config.exs

config :recipesite, Recipesite.Emails.Mailer,
  adapter: Bamboo.TestAdapter,
  api_key: "api-key"

but when I try to send an email I get an argument error

could not fetch application environment Recipesite.Emails.Mailer for application :recipesite because configuration at Recipesite.Emails.Mailer was not set

and even if I try to use Application.fetch_env!(:recipesite, Recipesite.Emails.Mailer) I get the same error, but calling something that came defined when I created the phoenix app works, like Application.fetch_env!(:recipesite, Recipesite.Repo).

Why can I fetch the preexisting config definitions but not my own? What am I doing wrong here?

Thanks!