Short answer. A bounce is a returned message. A hard bounce (5xx code) is a permanent error: the address does not exist, the domain is dead — suppress that address immediately. A soft bounce (4xx code) is temporary: full mailbox, greylisting, rate limit — retry it later. Ignoring hard bounces quickly wrecks sender reputation and deliverability.
How hard and soft differ
- Hard bounce: 5xx, permanent rejection. Suppress the address forever.
- Soft bounce: 4xx, temporary rejection. 3–5 retries with backoff are fine.
- A run of soft bounces to one address → promote to hard after a threshold.
Every unhandled hard bounce tells the provider "this sender ignores its list." A few percent of those and you land in spam.
SMTP code table
| Code | Type | Meaning | Action |
|---|---|---|---|
| 550 5.1.1 | Hard | Address does not exist | Suppress immediately |
| 550 5.7.1 | Hard | Rejected by policy / block | Suppress, check reputation |
| 552 5.2.2 | Hard/Soft | Mailbox full | Retry, promote to hard if repeated |
| 421 4.7.0 | Soft | Temporarily unavailable / throttling | Retry with backoff |
| 451 4.3.0 | Soft | Local server error | Retry later |
| 450 4.2.1 | Soft | Greylisting | Retry in 5–15 min |
How to read a code
The first digit sets the class: 4xx — temporary, 5xx — permanent. The extended code (e.g. 5.1.1) refines the reason per RFC 3463.
550 5.1.1 <user@example.com>: Recipient address rejected: User unknown
421 4.7.0 Try again later, mail server temporarily rejected messageHandling logic
- Receive a delivery status notification (DSN) or the in-session SMTP code.
- Classify: 5xx → hard, 4xx → soft.
- Hard → suppression list, stop sending.
- Soft → retry queue (3–5 attempts, exponential backoff).
- Soft over the retry threshold → promote to hard.
The suppression list
This is the list of addresses you must not mail. It protects reputation: you stop hammering dead addresses. Keep it on the ESP side and sync it across campaigns.
Metrics to watch
- Bounce rate — share of returns; keep hard bounces under 2%;
- Soft-to-hard conversion — how many temporaries became permanent;
- Repeat soft — addresses that consistently return 4xx.
Where enterno helps
enterno does not send campaigns and does not process bounces for you — your ESP does. But we help remove the causes: /email-check validates SPF/DKIM/DMARC (a frequent cause of 550 5.7.1), and /monitors watches your MX availability and SMTP ports 25/465/587 so you learn about 4xx spikes from server outages in time. Alerts go to Telegram, Slack, email or webhook.
FAQ
Can I retry a hard bounce?
No. 5xx is permanent; retrying only harms reputation.
How many retries for a soft bounce?
Usually 3–5 with growing backoff; then promote the address to hard.
What is greylisting?
A temporary rejection (450 4.2.1) to filter spam bots. A legitimate server retries and passes.
Where do I get the bounce reason?
From the DSN report and the extended SMTP code; a full breakdown of causes is in a separate article.
Next: read why emails bounce, check SPF/DKIM/DMARC and set up mail-server monitoring.