"The email platform we needed didn't exist. So we built it."
Jesse Hanley, Founder at BentoTransient (4xx)
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.
4 responses for SMTP 421
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
Gmail blocked this for low domain reputation. Check Postmaster Tools, fix authentication, and pause volume until reputation recovers.
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
Gmail blocked this because the content or links look suspicious. Clean the template, remove risky URLs, and retry at lower volume.
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 rate-limited your IP for unusual unsolicited traffic. Slow sends, scrub engagement, and review Postmaster Tools before ramping again.
4.7.32 This mail has been rate limited because there is no DMARC alignment
Gmail deferred this because DMARC alignment is missing. Align SPF or DKIM with your From domain so DMARC passes, then resend.
3 responses for SMTP 421
smtp;421 Service not available, closing transmission channel
Yahoo is not accepting mail on this connection right now. Treat it as temporary and retry with backoff.
smtp;421 Max message per connection reached, closing transmission channel
Yahoo closed the connection after hitting the max messages per connection. Send fewer messages per SMTP session and retry.
smtp;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
Yahoo deferred this for volume spikes, complaints, or weak reputation. Check complaint and bounce rates, then slow sending until it clears.
1 responses for SMTP 421
smtp;421 4.3.2 Service not available [outlook.com]
Outlook is not accepting mail on this connection right now. Retry with backoff. Escalate only if it keeps repeating.
1 responses for SMTP 421
smtp;421 4.4.2 mail.yandex.net Error: timeout exceeded
Yandex timed out while receiving the message. Retry with backoff. If it repeats, reduce concurrency to that host.
2 responses for SMTP 421
smtp;421 4.7.0 me.com Error: too many errors
iCloud closed the session after too many SMTP errors. Fix the underlying reject, then reconnect and retry.
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
iCloud deferred this mailbox because it is receiving mail too quickly. Wait and retry. Spread sends if many land on the same address.
1 responses for SMTP 421
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]
Exchange Online hit its concurrent connection limit for this forest. Lower connection concurrency and retry.
2 responses for SMTP 421
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)
Proofpoint could not verify the recipient and deferred delivery. Retry later. If it keeps failing, confirm the address with the recipient admin.
smtp;421 4.1.1 <email@example.com>: Recipient address rejected: unverified address: Address verification in progress
Proofpoint is still verifying this recipient address. Wait and retry. Do not treat it as a hard bounce yet.
2 responses for SMTP 421
smtp;421 Temporary failure, please try again later.
Barracuda returned a temporary failure, often quoting a downstream bounce. Fix the issue for the quoted recipient before sending that group again.
smtp;422 License Expired
Barracuda returned a temporary failure, often quoting a downstream bounce. Fix the issue for the quoted recipient before sending that group again.
1 responses for SMTP 421
smtp;421 Service Temporarily Unavailable
Email Security.cloud is temporarily unavailable. Retry with backoff. Investigate if the same response keeps repeating.
Response catalog adapted from the MIT-licensed SMTP Field Manual by ActiveCampaign / Postmark.
