We sent an outbound email on December 27th and got an unusually high soft bounce rate. After diving into the bounces, we found that most of them were coming from internal employees who qualified for the campaign. Those employees had Out of Office notifications for that day. Has anyone else seen out of office notifications cause soft bounces?
Hi Sanford,
Thanks for getting back to me. I was not able to replicate this with my own OoO. Here are a few examples from that campaign: 
Well, these are different.
The second one (the [oob]) could have been the Out of Office response mistaken for a post-delivery bounce.
The first one is a real SMTP + DNS error, meaning during that time Marketo couldn't connect to your domain's mailserver(s). Since this happened some time ago and (presumably) isn't reproducible now, it wouldn't be useful to check your current domain health. But if you see this again we can dive deeper.
An OoO that isn't recognized as such can appear to be a general out-of-band (i.e. post-delivery) bounce.
Can you show the details/category for a couple of these? Also, can you replicate the behavior by setting up your OoO now?