What it looks like in your logs
450 4.7.1 Client host rejected: cannot find your hostname, [203.0.113.10]What does 4.7.1 mean?
Enhanced code 4.7.1 combines a temporary class with the security or policy subject and the "delivery not authorized" detail. The receiver has made a policy decision against this message right now but is inviting a retry. RFC 3463 notes that X.7.1 is useful only as a permanent error, yet the temporary form is one of the most common deferral codes in real mail logs.
Several unrelated checks surface as 4.7.1. Greylisting servers use it on first contact. Postfix uses it when it cannot find a hostname for your IP. Providers use it to throttle senders whose reputation is under review. Some servers defer with 4.7.1 when an SPF or DMARC lookup hits a temporary DNS error. Each of these has a different fix, so the text after the code is where you start. The same code from two servers can mean two unrelated things.
Microsoft 365 returns 451 4.7.1 "Please try again later" and similar 4.7.x deferrals when it limits a sending IP. Treat a rising 4.7.1 rate as an early warning that tends to come before 5.7.x rejections.
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 |
| 4.7.1 | ✓ this code |
Common causes
- Greylisting deferred an unfamiliar sender, IP and recipient combination.
- The sending IP has no PTR record, or the HELO name does not match it.
- Volume or concurrent connections to one provider exceeded its limits.
- The IP is on a blocklist that the receiver uses for deferral rather than rejection.
- An SPF or DMARC lookup returned a temporary DNS error.
- The receiver defers low-reputation or unauthenticated senders while it re-evaluates them.
How to fix 4.7.1
- Let the MTA retry; if the next attempt delivers after about 15 minutes, it was greylisting and needs no action.
- Check your PTR with the reverse DNS checker and make the HELO/EHLO hostname match it in both directions.
- Check the sending IP with the blacklist checker.
- Confirm your SPF and DKIM records resolve quickly with the SPF checker and DKIM checker.
- Reduce concurrent connections and hourly volume to the affected provider, then ramp up slowly.
- Make sure your platform classifies 4.7.1 as a deferral and never adds the address to a suppression list.
4.7.1 questions
What does SMTP error 4.7.1 mean?
It means the receiving server is refusing your message temporarily for a security or policy reason and wants you to retry. Common causes are greylisting, missing reverse DNS, rate limits and temporary reputation checks. The address is not the problem.
How do I fix 450 4.7.1 client host rejected cannot find your hostname?
Your sending IP has no reverse DNS, or it does not match. Ask whoever owns the IP to set a PTR record to a hostname in your domain, point that hostname's A record back to the IP, and use it as your HELO name. Verify with the reverse DNS checker.
What is the difference between 4.7.1 and 5.7.1?
4.7.1 is temporary and should be retried, and addresses should stay on your list. 5.7.1 is a permanent refusal that needs a fix before you send again. Many 5.7.1 blocks are preceded by weeks of rising 4.7.1 deferrals. Handle them differently in your bounce processing.
Does 4.7.1 mean my mail is being greylisted?
Sometimes. If the text mentions greylisting, or the message delivers on the next retry a few minutes later, that is what happened. If 4.7.1 persists across retries, it is more likely rDNS, a blocklist or a reputation limit. See 450 for more on greylisting.