What’s the difference between “rejected” and “bounced”?

Rejected and bounced describe different failure timing:

Rejection happens during the SMTP conversation:

The receiving server evaluates the message and refuses it immediately. The sender knows instantly that delivery failed. No separate notification needed because the sender was present for the refusal.

Bounce happens after acceptance:

The receiving server accepts the message into its queue. Later processing discovers a problem (user does not exist, quota exceeded). A separate bounce message (DSN) is generated and sent back to the sender.

Why it matters:

Rejections are immediate and reliable. Bounces can be delayed, lost, or misrouted. Rejections are preferred for efficiency and clarity.

Rejection is "no entry" at the gate. Bounce is "return to sender" after the package was accepted.