450 4.2.1 The user you are trying to contact is receiving mail at a rate that prevents additional messages from being delivered
The limit here belongs to the recipient, not to you. One Gmail mailbox can only accept so many messages per minute and per day, and this address has hit it. Gmail asks you to retry; it is not a judgement about your mail.
What the server replies
450-4.2.1 The user you are trying to contact is receiving mail at a rate that 450-4.2.1 prevents additional messages from being delivered. Please resend your 450-4.2.1 message at a later time. If the user is able to receive mail at that 450-4.2.1 time, your message will be delivered. For more information, go to 450 4.2.1 https://support.google.com/mail/?p=ReceivingRatePerm
- Sent by
- Gmail
- Type
- TemporaryRetry helps
- About
- Rate limits and quotas
- Enhanced code
- 4.2.1
Who sends it
Gmail inbound servers at RCPT TO, for a specific recipient address. Group addresses, shared inboxes and addresses used by monitoring systems reach the limit most often.
Why it happens
- A monitoring or alerting system is sending hundreds of messages to one address during an incident.
- A mailing list or forwarder funnels many senders into one Gmail account.
- The address is under a mail-bombing or subscription-bombing attack and everyone's mail is being deferred alongside the attacker's.
- Your own application has a retry loop sending the same notification repeatedly.
How to fix it
Let your MTA retry
This is a 4xx reply; a normal retry schedule delivers the message once the recipient's rate drops. No action is needed for a one-off.
Batch what you send
If you are the source of the volume, group alerts into digests. Gmail's per-recipient limits are generous for people and tight for firehoses.
Tell the recipient
If the volume is not yours, the recipient should look at what else is hitting the mailbox and filter or unsubscribe it; a Workspace admin can raise limits for a group address by converting it to a Google Group.
Check it yourself
These free tools show the records and connections behind this code for your own domain or server.
Related codes
- 421 4.7.0 Our system has detected an unusual rate of unsolicited mail originating from your IP addressGmail is throttling the sending IP.
- 452 4.2.2 Mailbox full / over quotaThe recipient's mailbox has no room for the message.
- 451 4.7.500 Server busy, please try again laterMicrosoft is throttling your IP.
Sources
Fewer bounces to look up.
Faivelo queues temporary failures and retries them on a schedule receivers accept, then tells your application once, in plain words, if a message finally cannot be delivered.
Free tier included. No card needed.
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.