When a participant sends an email from their Participant Center to a target, and if that target has an out of office email set, the out of office reply email will be sent to the "Notif_From" address and not the "reply to" address.

This is an unfortunate effect of two separate things.  Due to SPF and DMARC requirements, Luminate sends emails on behalf of a user from an accepted SPF record (i.e. info@yourorg.org).  This is done in order to show we are a "safe sender" and are not spamming or emailing on behalf of people without their consent.

Read more on DMARC here

On the other side of the issue is Outlook, which is sending the out of office replies to the Notif_From address, because the "best practice" when replying to a system email is to reply to the system address, rather than the reply to address.

You can read more on that here and here

"The Reply-To field SHOULD NOT be used as the destination for automatic responses from Personal or Group Responders. In general, this field is set by a human sender based on his/her anticipation"

What this comes down to is that both sides are operating in the best way that can be done, and the side affect is that the out of office emails are going to the notif_from address, because there is no way to prevent this from happening on the Luminate side, and on the Outlook side the replies are working as expected.