Billing is based on the number of messages that goes through MailChannels Relay servers, this includes spam messages.
MailChannels is not only an SMTP Relay Service, but a service that analyzes the content of the messages sent, as a value added service, preventing that no SPAM is delivered to the receipients, hence why all messages are charged, including those flagged as SPAM.
MailChannels system has capabilities to reduce bursts in volume that may result from spam attacks. If your account starts sending an unusual volume of messages, it your account will be limited.
We recommend you to enable notifications in your MailChannels Cloud Account, and setup Webhooks to automate certain functions, e.g: suspend a cPanel or Email Account.
Technical details for the billing side:
Sent - SPAM detected = Queued
Queued - Bounces = Delivered
To make it simple to understand your dashboard and billing, you'll be billed for the total "Sent" messages.