What it looks like in your logs
421-4.7.0 [203.0.113.10] Our system has detected that this message is suspicious due to the very low reputation of the sending IP address. To best protect our users from spam, the message has been blocked. - gsmtpWhat does 4.7.0 mean?
Enhanced code 4.7.0 is the temporary version of other or undefined security status. The receiver has a security or policy concern and is deferring the message instead of rejecting it. Your server retries, and the message may go through later, but the receiver is telling you it does not fully trust this traffic. How you respond to repeated deferrals matters more than any single one.
Gmail is where most senders meet it. Gmail returns 421 4.7.0 when a message looks suspicious because of very low IP reputation, when a connection needs TLS, and for generic "try again later" deferrals. Occasional 4.7.0s are harmless. A steady share of them from Gmail is a leading indicator that reputation is sliding and permanent rejections may follow. Watch the share of deferrals per send, not just the final delivery rate.
Microsoft 365 uses 4.7.0 and neighboring 4.7.x codes for temporary reputation limits, especially on new IPs. Yahoo defers with 421 4.7.0 and tags such as TSS04, which means messages were temporarily deferred because of unexpected volume or user complaints.
Soft or hard bounce?
| Soft bounce (4xx) | Hard bounce (5xx) | |
|---|---|---|
| Nature | Temporary | Permanent |
| Sender behaviour | Queue and retry with backoff | Stop, suppress the address |
| Reputation risk | Low, unless it repeats for days | High above a 2% bounce rate |
| 4.7.0 | ✓ this code |
Common causes
- The sending IP has low or no reputation with the provider.
- A new or cold IP started sending at volume.
- Recent complaints or spam-trap hits damaged reputation.
- A sudden volume spike triggered protective throttling.
- The receiver required TLS and your server did not start it.
- The IP was recently delisted from a blocklist and reputation has not recovered yet.
How to fix 4.7.0
- Confirm your MTA retries 4.7.0 and does not suppress addresses on it.
- Read the text: if it mentions TLS, enable STARTTLS on outbound connections.
- Check the IP and domain with the sender reputation checker and review Google Postmaster Tools for reputation and spam rate.
- Cut volume to the affected provider and send to your most engaged recipients first.
- Warm new IPs gradually over two to four weeks using the IP and domain warmup guide.
- Confirm SPF, DKIM and DMARC pass with the DMARC checker and related checkers.
4.7.0 questions
What does Gmail error 421 4.7.0 mean?
It means Gmail temporarily deferred your message for a security or policy reason, most commonly low IP reputation. The message stays in your queue and is retried. If you see it often, Gmail is losing trust in your traffic and you should reduce volume and look at complaints.
Is 4.7.0 the same as being blacklisted by Gmail?
How long do 4.7.0 deferrals last?
They last as long as the reputation problem does. Individual messages often deliver on a later retry. The pattern usually improves within days to a couple of weeks once you lower volume, remove unengaged recipients and bring complaints down. Keep watching Postmaster Tools while it recovers.