I use 1000-2000 mails / month, I registered package with 40k mails. Someone put 400k spam emails to the system. Maybe the API keys is lost somehow. The Sendgrid helped to check the case and notified me to secure the account (I did not check it for 2 year). When I complaint about the monthly payment, Sendgrid checked the case and refund to customer. I'm appreciate about this.