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Delivery
What is Hard bounce?
A permanent delivery failure, such as an address that doesn't exist.
How it works in practice
SMTP reports it with a 5xx code, for example 550 5.1.1 for an unknown recipient. A sender should stop mailing the address immediately; continuing to send to dead addresses is one of the quickest ways to damage a reputation.
The common mistake
Retrying hard bounces or leaving them on the list.
Check it yourself
Related terms
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