mailer.rb

##################### mailers/welcome.rb
class Mailers::Welcome
  include Hanami::Mailer

  templates

  from    'noreply@bookshelf.org'
  to      'user@example.com'
  subject 'Welcome to Bookshelf'

  def greeting
    'hello, world'
  end
end

##################### mailers/templates/welcome.html.erb
<h2><%= greeting %></h2>

##################### welcome_spec.rb
require 'spec_helper'
# require your mailer here
# require_relative '...'

RSpec.describe Mailers::Welcome do
  before do
    Hanami::Mailer.deliveries.clear
  end

  it 'delivers email' do
    Mailers::Welcome.deliver
    mail = Hanami::Mailer.deliveries.last

    expect(mail.body.encoded).to include('hello, world')
  end
end