We’re having a similar experience. I got a dedicated HubSpot IP to try and get more of my emails delivered. In Mid-May, the number of unengaged emails shot through the roof! We have put the unengaged lists through MailChimp and had better success. In addition, I use a system called MailShake for cold email outreach (it’s designed for that) and I’m getting as good a response as my opt-in lists. The MailShake system has a bunch of rules for getting into inboxes and out of spam folders such as text only, no images, 2 links max, etc. I think email filters have changed and are blocking emails from MailChimp, HubSpot, etc. and blocking emails with images, too many links, etc.
We’re also retargeting with social media ads and RollWorks ads (B2B arm of AdRoll) which is helping.