Mailer specs
By default Mailer specs reside in the spec/mailers folder. Adding the metadata
type: :mailer to any context makes its examples be treated as mailer specs.
A mailer spec is a thin wrapper for an ActionMailer::TestCase, and includes all of the behavior and assertions that it provides, in addition to RSpec’s own behavior and expectations.
Examples
require "rails_helper"
RSpec.describe Notifications, type: :mailer do
  describe "notify" do
    let(:mail) { Notifications.signup }
    it "renders the headers" do
      expect(mail.subject).to eq("Signup")
      expect(mail.to).to eq(["to@example.org"])
      expect(mail.from).to eq(["from@example.com"])
    end
    it "renders the body" do
      expect(mail.body.encoded).to match("Hi")
    end
  end
end