What’s a “synchronous” vs “asynchronous” bounce?

Synchronous bounces occur during the live SMTP conversation:

The sending server is connected and waiting for response. The receiving server immediately rejects with error code. No separate notification needed. Instant feedback on delivery failure. Also called "rejections" or "inline bounces."

Asynchronous bounces arrive later as separate messages:

The receiving server initially accepts the message (250 OK). Later processing discovers a problem. A Delivery Status Notification (DSN) is generated. The DSN is sent to the Return-Path address. Can arrive minutes, hours, or even days later.

Implications:

Synchronous is more reliable and immediate. Asynchronous can be lost, delayed, or formatted inconsistently. Synchronous is preferred from a deliverability perspective.

Synchronous is "no" in real-time. Asynchronous is a letter sent later saying "actually, no."