421 4.7.0 — Too many messages - try again later
You are being rate limited, not blocked
SMTP error 421 4.7.0 is a receiver-side throttle: the destination provider is accepting mail from you but not this fast. The 4.x class means temporary, so the correct response is to slow down and retry. It is an early warning that your volume or reputation is close to that provider’s tolerance.
What the server actually says
421-4.7.0 Try again later, closing connection. (MAIL) For more information, go to
421 4.7.0 https://support.google.com/mail/?p=DefectiveForwarding - gsmtpWhat you see
Intermittent failures to one provider that resolve on retry, concentrated during your heaviest sending periods. Often precedes harder 5.7.x rejections if volume keeps climbing.
Why it happens
- Sending to one provider faster than it will accept from a sender at your reputation level.
- Too many simultaneous connections to the same receiving server.
- Thin or declining domain reputation, which lowers the rate a provider will accept.
- A volume ramp that outpaced reputation building.
How to fix it
- Reduce concurrency to that provider. Fewer parallel connections, more spacing. Providers throttle per connection as well as per volume.
- Retry with backoff. A 4.x is retryable by design. Back off progressively rather than hammering the same rate.
- Treat it as a reputation signal, not just a rate one. The acceptable rate scales with reputation. Getting 421s at modest volume means reputation is the real constraint.
- Slow the ramp. If 421s appear while ramping, the ramp is ahead of the reputation it is supposed to be building. Hold volume until they stop.
What not to do
- Do not treat it as a bounce or suppress the recipient - the address was never evaluated.
- Do not keep the same send rate and rely on retries. The provider is asking for less, and ignoring it invites a 5.7.x.
- Do not stop the mailbox. It is a throttle; the mailbox is fine.
Frequently asked questions
Is 421 4.7.0 serious?
On its own, no - it is a temporary throttle. Persistently, yes: it means you are at the edge of what that provider will accept from you, and continued pressure tends to escalate to hard policy rejections.
Difference from 451 4.7.1?
Both are throttles, but the source differs. A 451 Ratelimit usually comes from your own outbound host; a 421 4.7.0 comes from the receiving provider, so it reflects how they see your reputation.
How MailPilot helps
MailPilot classifies this error automatically on every connected mailbox and tells you which side is at fault, so a throttle is never mistaken for a dead recipient and a credential failure never suppresses a healthy address. It also monitors real inbox placement and checks SPF, DKIM and DMARC on every mailbox, which is where most policy rejections start.
Related errors
- 451 4.7.1 — Ratelimit exceeded
- 550 5.4.5 — Daily user sending limit exceeded
- 550 5.7.1 — Message rejected on policy grounds
Last reviewed 2026-10-11 by the MailPilot team.