Hard bounce
Definition
A hard bounce is a permanent delivery failure indicating the recipient address does not exist or cannot receive mail, and it should suppress the address immediately and permanently.
Why it matters
Amazon SES reports these with bounceType Permanent; SendGrid reports a bounce with a 5xx status or a blocked classification.
A soft bounce is a transient failure — a full mailbox, a temporary server problem — and does not warrant immediate suppression. Repeated soft bounces do: three within 30 days is a common escalation threshold.