Firstly, I would check your technical set up.
Have you set up SPF and DKIM for your Email Deliverability?
If you're not sure whether this is correctly implemented, you can ask Marketo Support to check for you, or alternatively post your email domain here and I'll happily take a look for you.
It's unlikely to be completely down to bad data, as you mention your first send's rate was ok, so this is why I suggest checking the tech-side first. If that's all fine, then the next steps would involve checking blacklisting status or the spamminess of the content.
Let me know how your investigation goes!