Deliverability

Email bounce codes explained: hard bounces, soft bounces, and what to do about each

When an email bounces, the receiving server sends back a code that tells you exactly why. Read it correctly and you know whether the problem is a dead address, a full mailbox, or your own sending configuration. Read it wrong and you keep sending to a wall.

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What is an email bounce?

A bounce is the receiving mail server's way of saying "I will not accept this message." Every bounce carries a three-digit SMTP status code (like the ones in our SMTP error code guide) plus a short human-readable reason. The first digit tells you the most important thing:

4xx: soft bounce. Temporary. The server will retry. The message is not lost.
5xx: hard bounce. Permanent. The server will not retry. The address or the request is dead.

That single distinction drives everything else: how you clean your list, how you handle retries, and whether your sender reputation is at risk.

Hard bounces (5xx): stop sending to these

A hard bounce means the address or the request will never succeed. Keep sending to a hard-bounced address and you teach the receiving provider that your list is dirty, which drags your entire domain toward the spam folder.

Code	Meaning	Fix
550 / 5.1.1	User unknown / mailbox does not exist	Remove the address from your list permanently.
551 / 5.1.2	User is not a local recipient	Wrong domain in the address. Verify and remove.
552 / 5.2.2	Mailbox over quota (sometimes)	Retry later; if it persists, treat as hard.
553 / 5.1.8	Address rejected / syntax error	Fix the malformed address; remove if unfixable.
554 / 5.7.1	Relay denied / policy rejection	Usually your SPF/DKIM/DMARC or IP reputation. Run an audit.
550 5.7.26	Gmail: bulk sender rules not met	See our 550 5.7.26 guide.

Soft bounces (4xx): retry, then re-evaluate

A soft bounce is temporary. The right response is to retry on a schedule. The wrong response is to give up immediately (you lose a real customer) or to hammer the address (you look like spam).

Code	Meaning	Fix
450 / 4.7.0	Temporary failure / try again later	Retry with backoff (1h, 6h, 24h).
451 / 4.3.0	Mail server busy / DNS timeout	Retry later. Check your own DNS health.
452 / 4.2.1 / 4.2.2	Mailbox full / over quota	Retry daily for a few days, then pause.
421	Server rate-limited you	Slow your send rate; check warm-up volume.

Bounce-rate thresholds: when to stop

Receiving providers and ESPs watch your bounce rate (bounces ÷ sends) closely. The commonly cited guardrails:

< 2%: healthy. Keep sending.
2–5%: warning. Clean your list and check your authentication.
> 5%: danger. Pause the campaign, clean aggressively, and fix the root cause before you send again.
> 10%: your sender reputation is at serious risk. Stop, audit, and warm back up.

Most of the time a rising bounce rate is not a "bad list" problem. It is an authentication or reputation problem on your sending domain. That is the part you can fix in an afternoon.

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