code: @Nonnull
    public AS2ClientBuilder setMessageHandler (@Nonnull final IAS2ClientBuilderMessageHandler aMessageHandler)
    {
      m_aMessageHandler = ValueEnforcer.notNull (aMessageHandler, "MessageHandler");
      return this;
    }
docs: Set the message handler to be used by the {@link #verifyContent()} method.
  By default an instance of {@link DefaultAS2ClientBuilderMessageHandler} is
  used so this method should only be called if you have special auditing
  requirements.
  
  @param aMessageHandler
  The message handler to be used. May not be <code>null</code>.
  @return this for chaining