Sent a campaign yesterday to a global list of 5,000, but I have yet to receive a single OOO. I know I have a great deliverability rating and optimize my send times, but I've never sent a campaign and not receive a single OOO response. Does Marketo hold response to the reply-to address until the campaign has completed when using Send in Recipient Time Zone?
Ahhh yes! If the 'reply-to' and 'from' addresses are different, often times the OOO messages will go to the 'from' address. As I understand it, the 'reply-to' is when someone clicks 'reply' and responds that way - much like we're doing on this thread with the 'reply.' 
often times the OOO messages will go to the 'from' address. As I understand it, the 'reply-to' is when someone clicks 'reply' and responds that way - much like we're doing on this thread with the 'reply.'
That's right, the Reply-To: is intended to be for human-triggered (non-automatic) responses.
Note the From: is also supposed to be for human responses/replies only. There's an entire RFC that covers automatic responses and it recommends unequivocally that the Return-Path (i.e. SMTP envelope sender, like unique-verp-id@something.mktomail.com) be used for auto-responses, not the From: (nor Sender:, nor Reply-To:) address.
However, this is an example of where the best-laid plans of an international standards body cannot compete with the decisions made by software vendors! In practice, mailservers send to the From: anyway.
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