For SMTP messages, you may send to up to 1,000 recipients at a time. If you are sending messages to more recipients, simultaneous and subsequent connections are permitted.
For the API, there is no recipient limitation, but the JSON provided per API call must be less than 10MB. We strongly recommend smaller recipient batches for easier troubleshooting.
When sending messages to multiple recipients in a single SMTP message or API call, if you do not want them to see each other’s information, set the preserve_recipients option to false (API) or the value of the custom header X-MC-PreserveRecipients to False (SMTP).
You can also control this option for the account as a whole on the Sending Options page using the Expose The List Of Recipients When Sending To Multiple Addresses option. Select the box next to this option to allow recipients to see each other’s information in the To header, and deselect the box so that each recipient sees only their own information in the To header.