Why It Matters
Hard bounces are one of the fastest ways to damage your sender reputation. ISPs track your bounce rate closely — anything above 2% is a warning sign, and above 5% can trigger automatic throttling or blocklisting. Every hard bounce is a signal that you're not maintaining your list, and ISPs treat careless senders harshly.
How It Works
When you send to an address that doesn't exist, the receiving server returns a 5xx error code — a permanent failure. Your ESP should automatically flag the address and prevent future sends. Unlike soft bounces (temporary issues like a full inbox), hard bounces won't resolve themselves. The address is gone. Common causes: typos during sign-up ("john@gmial.com"), deactivated accounts, or domains that no longer exist.
Quick Tips
- Remove hard bounces after the first occurrence — never retry them; they won't magically start working
- Add real-time email validation to your sign-up forms to catch typos and fake addresses before they enter your list
- If you see a sudden spike in hard bounces, check whether a partner or integration is feeding you bad data