When a delivery fails, the following error can appear in the delivery logs: DLV-XXXX The count of message prepared (123) is greater than the number of messages to send (111). Please contact support.
Usually, this error means that there is a personalization field or block within the email that has more than one values for the recipient. A personalization block is being used and it is fetching more than one record for a particular recipient.
To solve this, check the personalization data used, and then check the target for recipients that have more than one entry for any of those fields. You can also use a Deduplication activity in the targeting workflow prior to the delivery activity to check there is only one personalization field at a time. For more information on deduplication, refer to this page .