SMTP 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 →

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

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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

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GoogleAll Google codes →

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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

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MicrosoftAll Microsoft codes →

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

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VerizonAll Verizon codes →

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smtp;421 Service not available, closing transmission channel

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

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smtp;421 4.7.0 [TSS04] Messages from 0.0.0.0 temporarily deferred due to user complaints - 4.16.55.1; see https://help.yahoo.com/kb/postmaster/SLN3434.html

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Spam filters services

BarracudaAll Barracuda codes →

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smtp;421 Temporary failure, please try again later.

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Office 365All Office 365 codes →

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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]

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ProofpointAll Proofpoint codes →

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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)

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

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SymantecAll Symantec codes →

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smtp;421 Service Temporarily Unavailable

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Other SMTP codes