What happens after a deferred message retries?

Each retry attempt leads to one of three outcomes:

Successful delivery:

The temporary condition resolved. The receiving server accepts the message (250 OK). The message is delivered to the recipient.

Another deferral:

The temporary condition persists. Another 4xx response is returned. The message goes back to queue for the next retry attempt.

Permanent failure:

The server returns a 5xx code, indicating permanent failure. Or retry limits are exhausted (typically 24-72 hours of attempts). A bounce message is generated for the sender.

Retry schedules:

Most servers use exponential backoff: 5 min, 10 min, 30 min, 1 hour, 4 hours, etc. Total retry period typically ranges from 24 to 72 hours.

Each retry is another attempt to reach port. Eventually the ship either docks or returns home.