While we can specify the reply-to address, IETF technical specifications RFC-4021 and RFC-822 (http://tools.ietf.org/html/rfc4021) do not require that replies go to this address. Whether out of office auto-response messages are sent to the From: address or the Reply-To: address is up to the recipient mail server and is not under Marketo's control. Provided the Reply-To: address is set in the headers correctly, and often it is, it is up to the recipient's server to honour the "reply-to". I tend to appreciate when I receive messages expressly advising "do not reply this email as it is not monitored". It may be not the most diplomatic introduction. However it sets expectations right from the outset.