Assuming I'm understanding you correctly you sent 11,000 emails in under 24 hours to 10-15 people? For a newly created account I could understand that raising a few red flags. Though that doesn't justify their support staff failing to respond to queries nor the immediate account termination.

I get the feeling they're having trouble with their sender reputation; one of our larger clients was recently switched to SparkPost from Mandrill ("it's too expensive!") and it's now a continual battle with delayed deliveries and mail being marked as spam. The issue never arose with Mandrill. (and yes, SPF and DKIM are correct)