assert_no_emails

Asserts that no emails have been sent.

def test_emails
  assert_no_emails
  ContactMailer.welcome.deliver_now
  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, &block