SMTP Error Code 421

The mail server encountered a temporary failure. If the command is repeated without any change, it might be completed. Mail servers can use temporary failures like this to keep untrusted senders at bay.

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Email service providers

AppleAll Apple codes →

smtp;421 4.7.0 me.com Error: too many errors

smtp;421 4.7.1 Messages to example@icloud.com deferred due to excessive volume. Try again later - https://support.apple.com/en-us/HT204137

Medium severity

GoogleAll Google codes →

smtp;421 4.7.0 [x.xx.xx.xx] Our system has detected that this message is suspicious due to the very low reputation of the sending domain. To best protect our users from spam, the message has been blocked. Please visit https://support.google.com/mail/answer/188131 for more information. - gsmtp

High severity

smtp;421 4.7.0 [x.xx.xx.xx] Our system has detected that this message is suspicious due to the nature of the content and/or the links within. To best protect our users from spam, the message has been blocked. Please visit https://support.google.com/mail/answer/188131 for more information. - gsmtp

High severity

421 4.7.28 [x.xx.xx.xx] Our system has detected an unusual rate of unsolicited mail originating from your IP address. To protect our users from spam, mail sent from your IP address has been temporarily rate limited. Please visit https://support.google.com/mail/?p=UnsolicitedRateLimitError to review our Bulk Email Senders Guidelines. - gsmtp

High severity

OutlookAll Outlook codes →

smtp;421 4.3.2 Service not available [outlook.com]

Low severity

Yahoo IncAll Yahoo Inc codes →

smtp;421 Service not available, closing transmission channel

Low severity

smtp;421 Max message per connection reached, closing transmission channel

smtp;421 4.7.0 [TSS04] Messages from 0.0.0.0 temporarily deferred due to user complaints - 4.16.55.1; see https://postmaster.yahooinc.com/error-codes

High severity

YandexAll Yandex codes →

smtp;421 4.4.2 mail.yandex.net Error: timeout exceeded

Spam filters services

BarracudaAll Barracuda codes →

smtp;421 Temporary failure, please try again later.

Low severity

smtp;422 License Expired

Low severity

Email Security.cloudAll Email Security.cloud codes →

smtp;421 Service Temporarily Unavailable

Exchange OnlineAll Exchange Online codes →

smtp;421 4.3.2 The maximum number of concurrent connections per resource forest has exceeded a limit, closing transmission channel. example.com. PRX8 [example.com]

ProofpointAll Proofpoint codes →

smtp;421 4.1.1 <email@example.com>: Recipient address rejected: unverified address: host example.com[x.xx.xx.xx] said: 450 4.1.1 <email@example.com>: Recipient address rejected: unverified address: lost connection with mail.example.com[x.xx.xx.xx] while sending RCPT TO (in reply to RCPT TO command)

smtp;421 4.1.1 <email@example.com>: Recipient address rejected: unverified address: Address verification in progress

Other SMTP codes

Built with

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