Hard bounce
A permanent delivery failure: the mailbox does not exist, the domain does not accept mail, or the server has rejected the recipient for good. Hard bounces are the ones that wreck sender reputation.
A permanent delivery failure: the mailbox does not exist, the domain does not accept mail, or the server has rejected the recipient for good. Hard bounces are the ones that wreck sender reputation.
What a hard bounce looks like
A hard bounce is a permanent rejection from the receiving server, signalled by a 5xx SMTP code. The enhanced status code says why:
550 5.1.1— the mailbox does not exist ("user unknown"). The most common one by far.550 5.1.2— the domain does not exist or has no mail server.550 5.7.1— rejected on policy or reputation grounds, not because the address is bad.
The last one matters: a hard bounce is not always a dead address. A 5.7.1 can mean your domain is the problem. Read the code before you delete a lead.
Reading the bounce message
The bounce your sequencer shows you is the receiving server's own wording, and it is more specific than the number. Gmail, for example, says the account "does not exist" for a 5.1.1, which is a data problem. A message mentioning your IP, your domain, a blocklist, or "policy" is about you, and re-checking the address will not fix it. Sort the two before you act: delete the lead in the first case, look at your sender reputation in the second.
What to do after one
Suppress the address immediately and permanently. Sending to it again is the clearest signal of an unmanaged list, and mailbox providers notice. Good sequencers do this automatically; confirm yours does.
If hard bounces cluster on one company's domain, the cause is probably a wrong email pattern. Check the domain's email format before resending, not after.
How to prevent them
Almost every 5.1.1 is avoidable, because it is a question the mail server can answer before you send. Verifying the mailbox asks that question first. See also bounce rate and the difference from a soft bounce.
Ready to try it on a real list?
A permanent delivery failure: the mailbox does not exist, the domain does not accept mail, or the server has rejected the recipient for good. Hard bounces are the ones that wreck sender reputation.
Get startedRelated
- Cold email & deliverability glossary
Plain-language definitions for the terms that come up in cold outreach and email deliverability: bounce rate, catch-all, SPF/DKIM/DMARC, sender reputation, and more.
- Bounce rate — cold email glossary
The share of sent emails a mail server refuses to deliver. A hard bounce means the mailbox does not exist; a soft bounce means it is temporarily unreachable.
- Soft bounce — cold email glossary
A temporary delivery failure: a full mailbox, a greylisting delay, or a receiving server that is down. Retrying can succeed; treating every soft bounce as a dead address thins a list for no reason.
- Complaint rate — cold email glossary
The share of recipients who mark your mail as spam. Mailbox providers weigh this more heavily than bounces. Even a fraction of a percent is enough to land a domain in spam.