When we first started our Marketo instance, I found that many of our emails were being rejected as spam. After reaching out for assistance, it was discovered that our emails were rejected due to the servers we were on (as new Marketo customers, I guess?) Our sending process/database info was reviewed, and we were moved to different servers with a 'better' reputation. Our staying on the better servers is dependant on our using best practices, not spamming, maintaining a high deliverability percentage and not hitting spam traps/honey pots, etc. But that change absolutely solved our deliverability issues, back in the day. Now our deliverablility percentage is a consistant 98 to 99%. I'm not sure if Marketo still sets up instances on different servers based upon reputation/performance, etc., but if you review the doc Matjaž Jaušovec referenced and don't see any actions you can take, it might be worth reaching out to your rep or support.
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