mailer_with_changed_namespace.rb

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

    templates

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

    def greeting
      'hello, world'
    end
  end
end

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

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

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


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

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