Hard bounces come back with an SMTP status code starting with 5. The text after the code tells you why. Common causes are an address that never existed, a person who left the company and had their mailbox removed, a typo in the domain, or a domain that no longer has MX records.
550 5.1.1 The email account that you tried to reach does not exist.
550 5.4.1 Recipient address rejected: Access denied.
What to do after a hard bounce
- Never send to that address again. Retrying a hard bounce only adds to your bounce rate.
- Suppress it everywhere, not just in the campaign that bounced, so another campaign does not try it next week.
- If several addresses at one company bounce, the company may have changed domain; look the people up again.
- If hard bounces climb across a list, stop the campaign and verify the rest of the list before you continue.
Hard bounce or policy block?
Some 5xx replies are not about the address at all. A message such as "550 5.7.1 Message rejected due to policy" means the server refused your mail, often because of reputation or authentication. Treat those as a deliverability problem: check SPF, DKIM and DMARC, slow down and look at your sender reputation.
In ZicReach, an address that is rejected is marked bounced in every campaign in the workspace and suppressed, without blaming the mailbox, and a campaign pauses on its own when bounces pass its limit (5% by default). Verifying addresses first in the AI Leads Engine is the best way to avoid hard bounces at all.