Message Audit Log Details shows that delivery has failed. The delivery details show a message in the format:
### #.#.# Response message
550 5.7.1 message rejected due to content restrictions
The Delivery Details show a response from the next SMTP hop when trying to delivery the message. If the details do not show [internal], then the error message is originating from a different Mail Transfer Agent (MTA), usually either an internal server for inbound or an external server for outbound.