Thank you for coming into the Community. The billing is based off the total number of contacts in your account over the course of a billing period, however you can send to those contacts as many times as you want during that period. There's no limit to number of emails per day, and no limit to the number of campaigns you can create.
With any email provider, the delivery rate is going to depend more on the contacts being used than any other single factor. However, we do have some best practices that you can find here to improve that delivery rate. You can always contact us and we'll be happy to help with any deliverability issues that you might be having.
Our Terms and Conditions can be found here, and it will list any reasons that your account may be shut-down or deactivated.