What needs to be checked on the server to ensure that the message was successfully delivered?

Check the application sender server logs for entries of successful sending.

Check the message broker server logs (e.g., Apache Kafka, RabbitMQ) for entries of message receipt and routing.

Check the receiver application server logs for entries of message receipt and successful processing.

Check the database or data storage where the message processing should have recorded or changed state for relevant modifications.

When using delivery confirmation mechanisms at the application level (e.g., Acks in Kafka): Check the confirmation statuses in the sender system.

Check counters or metrics in monitoring systems (e.g., Prometheus, Grafana) related to message processing (number of sent/received/processed messages, processing time, errors).

For asynchronous processing: ensure that background tasks or processes responsible for message handling have completed successfully.

When using distributed transactions: check their status.