How are out of office messages handled?
How an 'out of office' message is handled is based on how the receiving server is set up to reply. Some receiving servers choose to soft bounce the message back to the bounce address and not deliver it. Our system treats that as a soft bounce.
Other receiving servers will reply directly to the 'from address' and notify them that the recipient is out of the office but the email will still be delivered. Our system does not ever see that reply so it's not considered a bounce.