How do retries work when a message can’t be delivered?

If a message cannot be delivered, the sending MTA places it back into its retry queue. It makes repeat attempts based on a schedule that starts with short retry intervals and increases over time.

If after the maximum retry window the message still cannot be delivered, the MTA sends a bounce to the Return Path address. This is the equivalent of a ship making repeated attempts to dock, then finally returning home with a report of failure.