421 4.7.28 — Unusual rate of mail from your IP (Gmail)
Retrying later can work — at a lower rate, not the same one.
The reply
421 4.7.28 Our system has detected an unusual rate of unsolicited mail.A temporary block. Gmail is refusing to take more mail from this IP right now, and the single most important thing is what you do next: slow down and it clears, keep hammering and it becomes permanent.
What it means
A 4xx is a deferral — the message is not lost and retrying later is correct behaviour. What the 4.7.28 adds is that this is a reputation-driven rate limit rather than a busy server, so the right retry is slower and gentler than your normal schedule. Gmail is explicitly asking you to reduce the rate; a sender that answers by retrying at full speed is telling it the limit was needed.
Why it happens
- Too many connections or messages per connection, too fast, from one IP.
- A volume spike from an IP with a short or thin history.
- Poor IP reputation, which lowers the rate Gmail is willing to accept.
- Several senders sharing one IP, where somebody else's traffic set the limit.
- Retrying a deferral aggressively, which deepens the throttle instead of clearing it.
What to do
- Slow down — fewer simultaneous connections, fewer messages per connection, and a longer gap between them.
- Back off exponentially rather than retrying on a fixed timer. Treat every 4.7.x as an instruction to reduce the rate for this receiver, not just to wait.
- Check IP reputation in Postmaster Tools. A limit this tight usually means reputation, not throughput.
- Warm a new IP gradually instead of starting at full volume.
- Spread a large send over hours rather than minutes.
Questions
Is my mail lost?
No. A 421 is temporary and a correctly configured sender will retry. It only becomes a lost message if it sits deferred until the retry lifetime runs out.
How long should I wait?
Start at several minutes and double each time. What matters more than the interval is that the overall rate to Gmail goes down and stays down until the deferrals stop.