class Mail::Message

def deliver!

Returns self

It still however fires off the interceptors and calls the observers callbacks if they are defined.

so use with caution.
This method bypasses checking perform_deliveries and raise_delivery_errors,
def deliver!
  inform_interceptors
  response = delivery_method.deliver!(self)
  inform_observers
  delivery_method.settings[:return_response] ? response : self
end