We were told by support that we were impacted because we were using a shared IP address. Are you saying that those with a dedicated IP are impacted as well? Marketo actually suggested we upgrade to a dedicated IP for $6000.
I have seen the problem recently as well. In my case I was told that someone who is sharing the IP with us did something that led to receiving servers 'rejecting/punishing' everyone using that IP. And apparently with the recent Malware, protection has ramped up a few levels.