Glossary
Hard bounce
Permanent delivery failure — mailbox does not exist, domain does not exist, blocked.
A hard bounce is an SMTP response in the 5.x.x range (550, 551, 553) indicating retries are pointless.
Typical causes: - 550 5.1.1 — User unknown / Recipient address rejected - 550 5.7.1 — Message blocked (recipient blocks your domain) - 553 5.1.3 — Invalid recipient address syntax
Sendersy automatically adds hard-bouncers to the suppression list — further sends to that address are blocked at the API level (POST /v1/emails returns an error).
This matters for reputation: ESP providers monitor hard-bounce rate. Exceeding 5% can lead to a temporary block of your IP/domain.