4.x.x transient failure
SMTP Error Code 421: Meaning, Causes and How to Fix It
SMTP 421 is a temporary (transient) failure.
The service is temporarily unavailable and the connection is being closed. The receiver is telling the sender to back off and reconnect shortly. It usually comes from rate limiting, a full queue, or a brief reputation check on an unfamiliar sender.
- Class
- 4xx transient
- Bounce type
- Soft bounce
- Highest severity
- High
- Responses on record
- 17
Apple
Barracuda
Email Security.cloud
Exchange Online
Google
Outlook
Proofpoint
Yahoo Inc
YandexSMTP 421 responses from email providers
How mailbox providers phrase a 421. Open a provider for the full text, severity and fix for every variation.
Apple2 responses
Google4 responses
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
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
Outlook1 response
SMTP 421 responses from spam filters
How spam-filtering gateways phrase a 421 rejection.
Barracuda2 responses
Email Security.cloud1 response
Exchange Online1 response
Proofpoint2 responses
smtp;421 4.1.1 <[email protected]>: Recipient address rejected: unverified address: host example.com[x.xx.xx.xx] said: 450 4.1.1 <[email protected]>: 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 protected]>: Recipient address rejected: unverified address: Address verification in progress
SMTP 421 questions
›Is SMTP 421 a hard bounce or a soft bounce?
It is a transient failure, so a soft bounce. The sending server queues the message and retries it automatically for a while (often 24 to 72 hours) before it gives up.
›What causes an SMTP 421 error?
The service is temporarily unavailable and the connection is being closed. The receiver is telling the sender to back off and reconnect shortly. It usually comes from rate limiting, a full queue, or a brief reputation check on an unfamiliar sender. The exact reason varies by receiver, which is why this page lists the specific 421 responses recorded for Apple, Barracuda, Email Security.cloud, Exchange Online, Google, Outlook.
›How do I fix an SMTP 421 bounce?
Read the enhanced status code and the message text, not just the number. Wording that names the recipient (user unknown, mailbox unavailable) means correct or remove the address. Wording that names authentication or reputation (SPF, DKIM, DMARC, blocklist) means fix that on the sending side, or clear the listing. Every response on this page has its own guidance.