Hard bounce

A hard bounce happens when the receiving system permanently refuses a message. It's signalled by a 5xx SMTP reply, often with an enhanced status code: 5.1.1 (bad destination mailbox, the address doesn't exist), 5.1.2 (bad destination system, for example the domain can't be found) or 5.1.10 (recipient address has a null MX).

Typical causes are a person who left the company, a mistyped or guessed address, a domain that expired or stopped receiving mail, or a name that never existed. Not every 5xx is about the address, though: 5.7.x codes usually mean the message was refused for policy or reputation reasons, which is a sender problem rather than a list problem.

Retrying a hard bounce won't help, and sending to such addresses again tells mailbox providers you don't maintain your list. Remove hard-bounced addresses immediately and add them to your suppression list so they don't sneak back in through a new import. Google's guidelines advise automatically unsubscribing recipients with multiple bounces.

Most hard bounces can be caught before you send. Skrap.ai's mailbox check over SMTP finds addresses the server rejects and marks them Invalid, at no credit cost. Run new lists through the free email verifier or a bulk upload, and use the email finder to look up a current address for contacts who've moved on.