Asserts that the number of emails sent matches the given number.

  def test_emails
    assert_emails 0
    ContactMailer.deliver_contact
    assert_emails 1
    ContactMailer.deliver_contact
    assert_emails 2
  end

If a block is passed, that block should cause the specified number of emails to be sent.

  def test_emails_again
    assert_emails 1 do
      ContactMailer.deliver_contact
    end

    assert_emails 2 do
      ContactMailer.deliver_contact
      ContactMailer.deliver_contact
    end
  end

[Source]

# File actionmailer/lib/action_mailer/test_helper.rb, line 25
    def assert_emails(number)
      if block_given?
        original_count = ActionMailer::Base.deliveries.size
        yield
        new_count = ActionMailer::Base.deliveries.size
        assert_equal original_count + number, new_count, "#{number} emails expected, but #{new_count - original_count} were sent"
      else
        assert_equal number, ActionMailer::Base.deliveries.size
      end
    end

Assert that no emails have been sent.

  def test_emails
    assert_no_emails
    ContactMailer.deliver_contact
    assert_emails 1
  end

If a block is passed, that block should not cause any emails to be sent.

  def test_emails_again
    assert_no_emails do
      # No emails should be sent from this block
    end
  end

Note: This assertion is simply a shortcut for:

  assert_emails 0

[Source]

# File actionmailer/lib/action_mailer/test_helper.rb, line 55
    def assert_no_emails(&block)
      assert_emails 0, &block
    end