Transact HTTP Maximum Recipient Count

This feature is disabled by default and will be enabled on a per org basis, as needed.
Transact offers a validation feature that is designed with large recipient counts in mind. When validation fails, the request will be written out as a file for the Transact Batch Processor (to be processed asynchronously) and the response will indicate that.
Solution
High recipient counts can cause issues with the Transact processing system. We recommend a maximum recipient count of 10, but some users submit requests with hundreds up to thousands of recipients.
Transact response if limit is exceeded:

• Status code: 2
• Error code: 14
• Error string: Recipient count exceeds max size of for synchronous send. Your request has been batched for asynchronous send via Batch Processor.

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Note:

It is important that you do NOT resend messages when you get error code 14. Otherwise, recipients will receive duplicate messages because the original request will be sent via batch vs synchronously via HTTP processor.