From Click Dimensions,
After investigation our teams have determined that some emails sent during the outage period have failed to send. The Amazon outage was first reported by Amazon at 9:37 AM PST on 12/7/2021 and was reported as resolved at 4:35 PM PST. During this time, some Email Sends sent by accounts in our US Data Center failed to send despite showing as "Sent" within D365/CRM.
We have put together guidance on how to identify and re-process Email Sends that were unable to send due to the Amazon Web Services outage that impacted our US Data Center on 12/7/2021. It can be found here: https://support.clickdimensions.com/hc/en-us/articles/4415953657613
We recommend customers review the linked guidance and determine if emails were impacted, and if the impacted email sends should be sent now.