- Hard bounce, defined
- A hard bounce is a permanent delivery failure: the receiving server rejected the message and will reject it again, usually because the mailbox does not exist.
The server answers with a 5xx code, most often 550, and names the reason: no such user, domain not found, or the mailbox is closed. Nothing about that changes if you send again next week.
Mailbox providers watch how many of these you generate. Repeatedly mailing addresses that have already returned a permanent failure is read as a signal you are not maintaining a list, and it is one of the faster ways to get filtered.
Every serious sending platform suppresses hard-bounced addresses automatically after the first failure. Verification moves that check before the send instead of after it.
How ZapBounce reports it
A permanent rejection comes back as invalid, with the server's reply code in smtp_code and a reason that says whether it was an unknown user (mailbox_not_found), a disabled account or a dead domain. A 5xx that reads as a block on our probe is unknown instead. Invalid results are billed, because we reached a definitive answer.
One Gmail refusal, read part by part
Say you send a product update to 8,000 subscribers on a Tuesday morning. Most receiving servers answer 250 and take the message. For 212 addresses the conversation ends differently, and Gmail's version of the refusal reads: 550 5.1.1 The email account that you tried to reach does not exist.
Each part of that line tells you something. The leading 5 marks the failure as permanent. Next comes 5.1.1, the enhanced status code, where the middle 1 points at the address and the last 1 says the mailbox wasn't found. After the codes you get free text written by whoever runs the server, which is why two providers describe an identical failure in different words.
Those 212 failures put the send at a 2.65% bounce rate. Your platform will suppress all of them without being asked. What it can't do is take back the signal, because every provider involved has already counted the attempts.
What those 212 addresses cost before the send and after it
Finding them beforehand is cheap. At ZapBounce's published price a 10,000-credit pack is $25, credits never expire, and an 8,000-address list leaves you 2,000 for next quarter. Anything that comes back unknown isn't charged.
Finding them afterwards costs something harder to price. A bounce rate that sits above 2% across several campaigns reads as an unmaintained list, and the penalty lands on mail to your good addresses too. You'll usually notice it weeks later as weaker opens at one provider, with no error message pointing back to the cause.
Cleaning won't take you to zero, and you should know that going in. Addresses on catch-all domains and at Yahoo can't be confirmed over SMTP, so a few hard bounces will still come from that group.
One more leak is worth closing. When you change sending platforms, export the suppression list and import it first, or the new tool will mail every dead address a second time.
Hard bounce: common questions
What bounce rate counts as a problem?
Under 2% is healthy. Above 5% and mailbox providers start treating the list as unmaintained. Those thresholds are about the whole send, not hard bounces alone.
Should I retry a hard bounce later?
No. The server has told you the address is permanently undeliverable, and mailing it again is the behavior that costs you reputation. Suppress it.
Can an address hard bounce and still be real?
Occasionally. An aggressive block on your sending domain can produce a 5xx that is about you rather than the recipient, which is why the response text matters.