Excellent points Sanford. That's exactly why I typically recommend distributing deliveries across the week if you can get away with it. This obviously depends on your content schedule and a number of other factors. As you know, every email system is unique, so the more you can be a "good citizen" email sender, the higher chance you have of inboxing and getting your message across.
We have the same issue. I have a record that is marked as an invalid recipient. However, the contact owner is able to communicate with this person via another sending platform.
We recently ran into a problem with contacts who use Symantec. We did a little research and came across this article on Symantec's website. I am not sure how new this is as part of their AntiSpam service, but it is clearly calling out Marketo sending servers. It just seems to be getting harder and harder to send using tools like Marketo to even communicate with our clients who are subscribed to receive communications. I have notified Marketo support about this and they have pushed it to the deliverability team. Looking forward to what they come up with.