What it looks like in your logs
421-4.7.28 [203.0.113.10] Our system has detected an unusual rate of unsolicited mail originating from your IP address. Mail sent from your IP address has been temporarily rate limited. - gsmtpWhat does 421 mean?
A 421 reply tells your server that the receiving side is not taking mail right now and is about to close the connection. It is a temporary failure, so nothing is wrong with the recipient address and the message stays in your queue. A healthy MTA waits, then tries again on a back-off schedule, and most one-off 421s clear on the second or third attempt without anyone noticing.
The trouble is that 421 covers two very different situations. Sometimes the receiver is genuinely busy, restarting or shedding load. More often at large mailbox providers it is a deliberate brake applied to your IP or domain because volume jumped, complaints rose or the IP has no history yet. The enhanced code and the text after it tell you which one you are dealing with, so always read past the first three digits.
Gmail pairs 421 with 4.7.0 or 4.7.28 when it rate limits suspicious or high-volume traffic. Microsoft 365 returns 421 4.3.2 when a server is unavailable and uses 421 with 4.7.x codes for throttling. Yahoo attaches bracketed tags such as TSS04, which means messages were deferred because of unexpected volume or user complaints. A steady stream of 421s from one provider is a reputation warning, not bad luck.
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 |
| 421 | ✓ this code |
Common causes
- You are sending more messages or opening more connections per minute than the provider accepts from your IP.
- The sending IP is new or cold and has no reputation history with the receiver.
- A sudden spike in volume, such as a large campaign after weeks of quiet, triggered automatic throttling.
- Recent complaints or spam-trap hits lowered your IP or domain reputation at that provider.
- The receiving server is overloaded, restarting or in maintenance.
- Greylisting is deferring an unfamiliar sender and recipient pair on first contact.
How to fix 421
- Confirm your MTA or ESP retries 421 automatically and does not count it as a bounce against the address.
- Read the enhanced code: 4.7.0 and 4.7.28 point to reputation or rate limits, 4.3.2 points to an unavailable server.
- Lower the number of concurrent connections and messages per hour to the affected provider, then ramp back up over several days.
- Check the IP and domain with the sender reputation checker and blacklist checker to rule out a listing.
- If the IP is new, follow a gradual schedule from the IP and domain warmup guide instead of sending at full volume.
- Watch Google Postmaster Tools and Microsoft SNDS for reputation and complaint trends while the deferrals continue.
421 questions
What does SMTP error 421 mean?
It means the receiving server is temporarily refusing mail and is closing the connection. Your message is not lost: it stays in the queue and your server retries later. One 421 is routine. Repeated 421s from the same provider usually mean you are being throttled for volume or reputation reasons and need to slow down.
Will my email be retried automatically after a 421?
Yes, any standards-compliant MTA or ESP treats 421 as a deferral and retries on a schedule, often for up to several days. If the message still cannot be delivered when the retry window ends, it becomes a bounce. Check that your platform does not suppress the address after a single 421, because that would remove valid recipients.
Why does Gmail keep returning 421 4.7.28?
Gmail uses 421 4.7.28 when it sees an unusual rate of unsolicited mail from your IP or domain. It is a reputation-driven rate limit, not a capacity problem. Reduce volume to Gmail, remove unengaged recipients, and check complaint rates in Postmaster Tools. See 4.7.28 for how to read which dimension Gmail is throttling.
What is the difference between 421 and 550?
A 421 is temporary: the server wants you to try again later and the message remains queued. A 550 is permanent: the server has refused the message and retrying will not change the answer. Treat 421 as a signal to slow down and investigate, and treat 550 as a reason to suppress or fix the root cause.