Guys, you're missing an ingredient here.
By specification, the preferred destination for auto-responses is the envelope sender, that is, the SMTP-level MAIL FROM address. This isn't the From: header, nor the Reply-To: header.
With Marketo emails, unless you have the special branded sender add-on to your subscription, the envelope sender is an address @*.mktomail.com. That's why you never get actual SMTP-level bounces to your header addresses (save for some scattered bounce-like traffic). It's also the reason SPF doesn't matter for most Marketo instances.
The auto-responder spec makes it very clear that "automatic responses" should go to the envelope sender (also referred to as the Return-Path: header, which should represent the exact same address, again not the From: or Reply-To:).
Problem: it's never been accepted across the board that Out Of Office messages qualify as "automatiic responses." So the disagreement is more about whether "automatic" (seen as "machine-generated, unsolicited") includes emails that required human intervention from the mailbox owner to author, may only be sent in a reserved time window, and (in most cases with OoO) are supposed to be read as if they are human responses. That is, we know person X is on maternity leave because they originally wrote the email. It's not AI figuring out which content to send.
None of this changes the fact that implementations differ, each side saying they're equally right. But you need to keep in mind there's an entire other mailbox that you have no access to that is routinely receiving automated messages.
You bring up a good point around the branded sender option. We're currently looking into this as we speak (since our deliverability is quite poor - even though Marketo shows 9x% delivered rate). So if we move forward with this, are you saying that one of our emailboxes will begin receiving all of the bounces that were, up until now, hidden?
So if we move forward with this, are you saying that one of our emailboxes will begin receiving all of the bounces that were, up until now, hidden?
Well, no (or not typically). Since Marketo still wants to process out-of-band bounces, they'll ask you to set a up new subdomain (say, send.avanade.com) and have the MX record for that domain go straight back to Marketo. So branded sender doesn't mean literally @avanade.com.
However, if you really wanted to see the bounce traffic, you could set the MX record for send.avanade.com to a server at your company to keep a log, then gateway the mail over to Marketo.
Since my company spends all our time handling Marketo replies and auto-responders, I can confirm Sanford's comment about many OOOs going to the From address, not the Reply-To address. We've seen this pattern across millions of email replies processed at Siftrock