Rspec example

subject_vs_it.rb

# кусок тестируемого класса
class MandrilMailer
  def self.deliver_welcome(whom, details={})
      template_name = case whom
                        when :painter then 'Welcome Painter'
                        when :homeowner then 'Welcome Homeowner'
                        else :none
                      end

      options = {
          email: details[:email],
          name: details[:name],
          subject: template_name,
          vars: [{
                    name: 'description',
                    content: "You are receiving this email because you registered as a #{whom.to_s} on EasyPaint."
                 }]
      }
      client.messages.send_template template_name, template_content, message(options)
    end

# test
require 'spec_helper'

describe MandrilMailer do

  describe '.deliver_welcome' do
    let(:name_and_email) { { email: 'test@test.pro', name: 'John Doe' } }

    # использовать всё это
    context 'send Wellcome email' do
      subject { MandrilMailer.deliver_welcome(:homeowner, name_and_email) }
      its(['status']) { is_expected.to eq('sent') }
    end

    # или просто это
    it 'send email' do
      expect(MandrilMailer.deliver_welcome(:homeowner, name_and_email)).to include('status' => 'sent')
    end
  end
end