A hard bounce means the address does not exist or has permanently rejected mail — invalid mailbox, blocked domain, or terminated account. Hard-bounced contacts should be suppressed immediately so you do not keep sending to dead addresses.
A soft bounce is a temporary failure: full mailbox, recipient server down, message too large, or a transient rate limit. Most platforms retry soft bounces a few times before giving up. Persistent soft bounces should be treated as hard bounces after a threshold (typically 3–5 attempts).
Sustained bounce rates over 2% will damage your sender reputation. The fix is list hygiene: never buy lists, always use double opt-in for sign-ups, and run a re-engagement campaign on inactive subscribers every 90 days. A high bounce rate after a list import is the single biggest unforced error in email marketing.