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.
Providers
Barracuda · Email Security.cloud · Exchange Online · Google · Outlook · Proofpoint · Yahoo Inc · Yandex · iCloud
- Barracuda↑
421 Temporary failure, please try again later.
This response is followed by the bounce received from one of the intended recipients of the message. You'll want to resolve the delivery issue for the quoted recipient before sending to this group of addresses again.
422 License Expired
The recepient domain uses a Barracuda mail filter and their license for that service is expired. They'll need to resolve their license issue with Barracuda to start to receive messages again.
- Email Security.cloud↑
- 421 Service Temporarily Unavailable
- Exchange Online↑
- 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]
- Google↑
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
Gmail has determined that the sending domain has a low reputation, so messages are now temporarily rejected. See Google's documentation on how to determine the source of this poor reputation and how to improve deliverability.
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
Gmail has determined that the content of the message has a low reputation. See Google's documentation on how to determine the source of this poor reputation and how to improve deliverability.
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
Gmail has determined your messages are unsolicted, so messages sent over this IP may be temporarily rejected. See Google's documentation for best practices on sending to Gmail to avoid this.
4.7.32 This mail has been rate limited because there is no DMARC alignment
The message is missing DKIM or SPF alignment for DMARC. Make sure your messages are passing DMARC. This message started appearing on January 21st, 2024 ahead of Gmail's new 2024 sender requirements.
- Outlook↑
421 4.3.2 Service not available [outlook.com]
This indicates that the domain is not currently accepting email.
- Proofpoint↑
- 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)
- 421 4.1.1 <email@example.com>: Recipient address rejected: unverified address: Address verification in progress
- Yahoo Inc↑
421 Service not available, closing transmission channel
This indicates that the domain is not currently accepting email.
421 Max message per connection reached, closing transmission channel
421 4.7.0 [TSS04] Messages from 0.0.0.0 temporarily deferred due to unexpected volume or user complaints - 4.16.55.1; see https://postmaster.yahooinc.com/error-codes
This could indicate one of few problems, either that your emails are generating an excessive amount of complaints from Yahoo users or Yahoo is seeing unsual traffic patterns from your mail servers. It could also mean that your email content contains objectionable content or show characteristics indicative of spam. Another possiblity is that your IP has a poor reputation
- Yandex↑
- 421 4.4.2 mail.yandex.net Error: timeout exceeded
- iCloud↑
421 4.7.0 me.com Error: too many errors
421 4.7.1 Messages to example@icloud.com deferred due to excessive volume. Try again later - https://support.apple.com/en-us/HT204137
The email address is currently receiving an email at a high rate. Sending email in the future will most likely be successful.