Fundamentals of Bounces

The "Return to Sender" notice. This section defines a "bounce" as a formal "Non-Delivery Report" (NDR) from a mail server, explaining why your message could not be delivered. It's the harbor master's official 'refusal of cargo' notice.

15 questions in this topic

What is a bounce in email? What’s the difference between a soft bounce and a hard bounce? What is a transient delivery failure? What’s the difference between “rejected” and “bounced”? How are bounce messages generated? What is a Non-Delivery Report (NDR)? What’s the difference between local vs remote bounce handling? What are SMTP 4xx vs 5xx error codes? What does it mean when an email is “deferred”? What happens after a deferred message retries? How do ESPs classify bounces? What is “retry logic” in bounce handling? How do repeated soft bounces become hard bounces? What’s a “synchronous” vs “asynchronous” bounce? How do bounces affect deliverability metrics?

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