I totally feel your frustration on this as it seems to come out of the blue!
Our issue was that we had a LOT of spam complaints registered after we sent out a re-permissioning email (to literally avoid that problem) in March/April last year. While I think the Smart Campaigns found this information using the bounce categories, our Marketo AM ran an DLVR Audit (using Splunk) which confirmed this and outlined that the bulk of the spam complaints (and bounces) were happening with Yahoo.com email addresses (includes Verizon domains too).
As these were temporary deferrals of delivery, we continued to send messages to these addresses which perpetuated the low delivery rate (around 72% instead of our usual 98%+); I moved all of these leads into Marketing Suspended status which immediately improved everything (and we continue to see our "normal" rates of delivery to this day).
I ended up removing these people from our Marketo database permanently as there was no point paying to keep them if we can't do anything with them.
Here are a couple of screenshots from the report we got (totalled about 10 pages with additional information) - it didn't give us anything brand new, but confirmed what we suspected in general:
High Level Deliverability Metrics:
For context, we don't buy lists and require all our leads to opt in to receive emails when they fill out a form on our website, so unfortunately this seemed to be a case of people forgetting who we are and marking us as Spam instead of simply unsubscribing (we also don't do giant email blasts to our entire database anyways so this was also a one-off to get people to re-confirm with us).
Hope that helps and feel free to ask more questions if you need!
Thank you @michellechopin and @raiashutosh for sharing this information!