group :development, :test do
  gem 'smtp_mock', require: false
end

# Public SmtpMock interface
# Without kwargs creates SMTP mock server with default behavior.
# A free port for server will be randomly assigned in the range
# from 49152 to 65535. Returns current smtp mock server instance
smtp_mock_server = SmtpMock.start_server(not_registered_emails: %w[[email protected]]) # => SmtpMock::Server instance

# returns current smtp mock server port
smtp_mock_server.port # => 55640

# returns current smtp mock server process identification number (PID)
smtp_mock_server.pid # => 38195

# returns current smtp mock server version
smtp_mock_server.version # => '1.5.2'

# interface for graceful shutdown current smtp mock server
smtp_mock_server.stop # => true

# interface for force shutdown current smtp mock server
smtp_mock_server.stop! # => true

# interface to check state of current smtp mock server
# returns true if server is running, otherwise returns false
smtp_mock_server.active? # => true

# returns list of running smtp mock servers
SmtpMock.running_servers # => [SmtpMock::Server instance]

# interface to stop all running smtp mock servers
SmtpMock.stop_running_servers! # => true

# Gemfile

group :test do
  gem 'rspec'
  gem 'smtp_mock', require: 'smtp_mock/test_framework/rspec'
end

# spec/support/config/smtp_mock.rb

require 'smtp_mock/test_framework/rspec'

# spec/support/config/smtp_mock.rb

RSpec.configure do |config|
  config.include SmtpMock::TestFramework::RSpec::Helper
end

# your awesome smtp_client_spec.rb

RSpec.describe SmtpClient do
  subject(:smtp_response) do
    described_class.call(
      host: 'localhost',
      port: smtp_mock_server.port,
      mailfrom: mailfrom,
      rcptto: rcptto,
      message: message
    )
  end

  let(:mailfrom) { '[email protected]' }
  let(:rcptto) { '[email protected]' }
  let(:message) { 'Email message context' }
  let(:expected_response_message) { '250 Custom successful response' }

  before { smtp_mock_server(msg_msg_received: expected_response_message) }

  it do
    expect(smtp_response).to be_success
    expect(smtp_response).to have_status(expected_response_status)
    expect(smtp_response).to have_message_context(expected_response_message)
  end
end

SmtpMock::TestFramework::RSpec::Interface.start_server  # creates and runs SmtpMock server instance
SmtpMock::TestFramework::RSpec::Interface.stop_server!  # stops and clears current SmtpMock server instance
SmtpMock::TestFramework::RSpec::Interface.clear_server! # clears current SmtpMock server instance