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Temporary failure

450 4.2.1 Gmail: recipient is receiving mail too fast

Quick answer

Gmail is deferring the message because this one recipient is receiving mail at a rate that prevents more messages from being delivered right now. The mailbox exists. It is a per-recipient rate limit.

Recommended action: Retry later

Common causes

  • Many messages sent to the same address in a short time, for example from a loop or a misconfigured automation.
  • A popular shared mailbox receiving a burst of mail.

What to do

Let your server retry later and check for a loop or duplicate sends to this address. Do not suppress the address: it is valid.

Provider note: RocketVerifier treats Gmail's 4.2.1 as greylisting: the check is retried automatically, up to three times about ten minutes apart, before any result is returned.

How to read 450 4.2.1

450 is the basic SMTP reply defined in RFC 5321. 4.2.1 is the enhanced status code from RFC 3463, which reads as class, subject and detail:

Class
4Temporary failure
Subject
.2Mailbox
Detail
.1Gmail: recipient is receiving mail too fast

Further reading

FAQ

Frequently asked questions

Is a 450 4.2.1 a soft bounce?

Yes. It is temporary, and the message is delivered once the recipient's incoming rate drops.

Related bounce codes

All bounce codes

Stop hard bounces before they start

Upload a list or call the API, and send only to addresses that can receive mail. Unknown results are always free.

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