**Update: Seems that the 4k+ delivery failures were due to a message-expired error. This is what the documentation has to say about the error, /message-expired – This category results when, within the lifecycle of the email message, it was not able to be delivered to the recipient because of sending limitations or transient errors. If this error occurs frequently, you should open a support ticket to resolve it./
But this explanation still does seem rather vague, does anyone know what transient errors might mean? Or what the sending limitations are?

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