Recipient(s) in an API call

You can only send to one recipient (To: or CC:) per API call.

Each API submit call equals one message to be delivered to a single recipient, and returns a unique ID to trace that message. The SubmitMessage object allows setting any combination of To and CC headers. In other words: if you send a message to two recipients with one in CC, you need to submit a message with those headers twice, one for both types of recipient addresses.

The best comparison here is with regular mail: email works the same way. The envelope (recipientAddress) determines where the message is delivered, the letter head may contain information about the intended recipients (but does not determine actual delivery).