Outbound Glossary
Hard bounce
A permanent rejection, almost always a dead address. Above roughly two percent it stops being a data problem and becomes a reputation problem.
A hard bounce means the address does not exist or the server will never accept mail for it. It is final, and sending again makes it worse rather than better.
Mailbox providers read a high hard bounce rate as proof you do not know who you are writing to, which is the clearest signal of a bought or scraped list. Verify before sending, remove bounces immediately, and treat a rising bounce rate as a reason to stop the campaign rather than a number to report at the end of it.