421 4.7.0 Our system has detected an unusual rate of unsolicited mail originating from your IP address
Gmail is throttling the sending IP. The message is not rejected for good; the server is told to come back later, and a well-behaved MTA will retry for hours or days. The same code also appears with "Try again later, closing connection" and "TLS required for RCPT domain", which are different temporary conditions.
What the server replies
421-4.7.0 [203.0.113.10 15] Our system has detected an unusual rate of 421-4.7.0 unsolicited mail originating from your IP address. To protect our 421-4.7.0 users from spam, mail sent from your IP address has been temporarily 421-4.7.0 rate limited. Please visit 421 4.7.0 https://support.google.com/mail/?p=UnsolicitedRateLimitError to review our Bulk Email Senders Guidelines.
- Sent by
- Gmail
- Type
- TemporaryRetry helps
- About
- Rate limits and quotas
- Enhanced code
- 4.7.0
Who sends it
Gmail inbound servers, usually right after RCPT TO or at the start of the session, when an IP's recent volume or complaint rate crosses a threshold. It is the first warning before the permanent 550 5.7.1 block.
Why it happens
- A sudden jump in volume from an IP that has little sending history with Gmail.
- A burst of complaints or bounces from a single campaign.
- Several unrelated customers sharing one IP, one of whom is misbehaving.
- Opening too many parallel connections to Gmail from one host.
- For the "TLS required" variant: the sending server offered no STARTTLS and the recipient domain enforces it.
How to fix it
Let the queue drain
Do not clear or resend the queue. Leave the messages to retry; Gmail lifts the limit on its own when the rate falls. Resending from a second IP spreads the damage.
Cut concurrency and rate
Limit connections to Gmail to a handful at a time and messages per connection to around twenty. Most MTAs have a per-destination transport setting for this.
Find the campaign that caused it
Compare the moment the deferrals began with your send log. Pause whatever list or customer started sending just before, and check its complaint rate in Postmaster Tools.
Warm up properly
For a new IP, start at a few hundred messages a day to Gmail and double every two or three days, sending to engaged recipients first.
Check it yourself
These free tools show the records and connections behind this code for your own domain or server.
- SMTP connection testerConnect to any SMTP server, check TLS and the login step, and see each stage with its timing.
- Email DNS checkerCheck MX, SPF, DKIM, DMARC and reverse DNS for any domain and get the exact records to add or fix.
- DMARC report analyzerDrop in a DMARC aggregate report and see who is sending as your domain and whether they pass.
Related codes
- 421 4.7.28 Our system has detected an unusual rate of unsolicited mail originating from your DKIM domainLike 421 4.7.0 but keyed to the domain rather than the IP: Gmail is deferring mail signed by, or claiming, your domain regardless of which server sends it.
- 550 5.7.1 Our system has detected that this message is likely unsolicited mailGmail's spam filter scored the message as spam before it reached a mailbox and rejected it at the SMTP level rather than placing it in the Spam folder.
- 450 4.2.1 The user you are trying to contact is receiving mail at a rate that prevents additional messages from being deliveredThe limit here belongs to the recipient, not to you.
- 421 4.7.0 [TSS04] Messages from IP temporarily deferred due to unexpected volume or user complaintsYahoo is slowing your IP down.
Sources
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Questions people ask
Other free tools
All tools- Email DNS checkerCheck MX, SPF, DKIM, DMARC and reverse DNS for any domain and get the exact records to add or fix.
- DMARC checkerLook up a domain's DMARC record, read every tag in plain words and see what to tighten next.
- DMARC record generatorBuild a valid DMARC record from a few choices and copy the host and value into your DNS.
- DMARC report analyzerDrop in a DMARC aggregate report and see who is sending as your domain and whether they pass.
- SMTP connection testerConnect to any SMTP server, check TLS and the login step, and see each stage with its timing.
- Email header analyzerPaste raw headers to see every hop, where the delay was, and whether SPF, DKIM and DMARC passed.
- Email address validatorCheck whether an address is well formed, whether its domain accepts mail, and whether it is disposable or a role account.