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 · GoDaddy · GoDaddy · GoDaddy · Google · Outlook.com · Proofpoint · Rackspace · Yahoo · 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]
GoDaddy↑
421 Connection refused, too many sessions from $(_ipsrc). Please lower the number of concurrent sessions. IB007 <https://www.secureserver.net/help/fix-rejected-email-with-a-bounce-error-40685?pl_id=3153&prog_id=3153#rlb

The connecting IP address has exceeded the maximum number of concurrent connections allowed to the server. This can happen if you are sending a large number of emails in a short period of time.

GoDaddy↑
421 Server temporarily unavailable. Try again later. IB101

The mail server encountered a temporary failure. Most likely it isn't anything to do with the session, but rather the server is overloaded or having issues. If you try again later, it should work.

GoDaddy↑
421 Temporarily rejected. Reverse DNS for $(_ipsrc) failed. IB108

The mail server encountered a temporary failure. Most likely it isn't anything to do with the session, but rather the server is overloaded or having issues. If you try again later, it should work.

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.

4.7.26 This mail has been rate limited because it is unauthenticated. Gmail requires all senders to authenticate with either SPF or DKIM. Authentication results: DKIM = did not pass. SPF example.com with ip: x.x.x.x = did not pass

The message did not pass SPF or DKIM. As of February 2024, Google requires SPF and DKIM to be correctly set up in order to be able to send emails to their customers. Contact your email provider to set up SPF and/or DKIM correctly.

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. As of February 2024, Google requires SPF and DKIM to be correctly set up in order to be able to send emails to their customers. Contact your email provider to set up SPF and/or DKIM correctly.

Outlook.com↑
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
Rackspace↑
421 Too many concurrent connections from this client (G2)

Too many concurrent connections from your IP. Throttle and reduce the number of connections your MTA is making.

Yahoo↑
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

You have exceeded the number of messages you can send in a single connection. If you do not control the MTA, you may need to ask your email provider for adjustments.

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.