Thanks Justin and Kristen.
We reached out to support and are looking into the following:
From Support: It sounds like your internal email server is blocking the HTML version of the email. I recommend reaching out to your IT team with this information and see if they have information as to why they are blocking the Marketo HTML emails from coming through. This is likely the issue since I was able to send myself tests from within your instance both HTML and text only, just fine. You mentioned your IT team whitelisted the Marketo IP, however the article I sent you has a range of IPs that Marketo uses, and perhaps they did not whitelist them all, or perhaps they did at some point and recently made a change.