[andwur]

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)

[posabsolute]

I must say I agree,

I'm surprised OP is not aware that's the kind of sending that can raise a lot of red flags, before doing something like this on a new account I would have contacted support to make them aware of this situation.

Specially on smaller senders, reputation is everything.