Wow! Something has definitely happened here. Unfortunately, even with that Smart List, there's probably several other factors at play. To get an idea of what's happening:
- What are you currently seeing for your hard/soft bounce rate?
- What does your standard deliverability rate look like?
- If you check Admin-> Email, are you seeing that SPF and DKIM are showing up as verified?
- Are you using standard Marketo email setup or do you have a special configuration, such as a dedicated IP?
- How did you make this particular email: are you using a template or setup that you've done in the past, or something new?