What is a hard bounce?
Also called: permanent failure, 5xx bounce
A hard bounce is a permanent delivery failure: the address does not exist, the domain does not exist, or the receiving server has permanently refused the mail. Hard bounces come back with a 5xx SMTP code and should never be retried; repeatedly sending to them is what damages sender reputation.
The distinction that matters is permanence. A hard bounce means the mail will not be delivered however many times you try, and mailbox providers treat continued attempts as a sign that you do not maintain your list.
Common causes: the mailbox was closed when someone left a company, the address was mistyped at signup, the domain expired, or the address never existed because it was guessed or invented to get past a form.
Mailbox providers do not publish a bounce threshold, but sending platforms do act on bounce rates and can pause an account that exceeds their limit. Verification is how you remove hard bounces before the send, because afterwards the damage is already recorded.
Example
550 5.1.1 The email account that you tried to reach does not exist. The first digit 5 means permanent; the enhanced code 5.1.1 means bad destination mailbox.How SimpleVerifier handles it
Addresses the server rejects outright are returned as invalid · Mailbox not found, and domains with no mail server as invalid · No mail server, so they can be removed before they ever bounce.
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Sources
By SimpleVerifier Team · Last reviewed 2026-10-13 · All 50 glossary terms