Deliverability
What is Hard Bounce?
A permanent delivery failure, usually because the recipient address does not exist.
A hard bounce is a permanent SMTP rejection, signaled by a 5xx response, indicating the message can never be delivered to that address, most often because the mailbox or domain does not exist. Unlike temporary failures, the sending MTA does not retry a hard bounce. A high hard-bounce rate tells mailbox providers your list is inaccurate, which directly damages sender reputation.
Why it matters for cold email
Cold lists are unconfirmed by nature, so invalid addresses are common, and pre-send verification to strip hard bounces is essential because Google and Microsoft throttle or block senders whose bounce rates climb past a few percent.
Related terms
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