Summary
The SMTP deferred message 'refused to talk to me: 421 4.7.0 Not allowed' typically signifies a temporary service unavailability on the receiving mail server. Potential causes include IP blocking, server overload, maintenance, greylisting, rate limiting, incorrect DNS or authentication configurations (SPF, DKIM, rDNS), exceeding sending limits, firewall issues, or the recipient server being too busy. While retrying the email later is crucial, persistent errors warrant investigation of server configurations and contacting support.
Key findings
- Service Unavailability: The 421 error primarily indicates the receiving mail server is temporarily unavailable.
- Configuration Issues: Incorrect DNS settings, SPF/DKIM records, or reverse DNS can trigger this error.
- Anti-Spam Measures: Greylisting, IP blacklisting, and exceeding sending limits can result in a 421 error.
- Network Problems: Firewall interference or network issues can disrupt SMTP connections.
- Rate Limiting: The recipient server might be limiting the number of emails accepted.
Key considerations
- Retry Sending: Always retry sending the email after a short delay as the issue may be transient.
- Check Configurations: Verify DNS settings, SPF/DKIM records, and reverse DNS configuration for correctness.
- Review Sending Limits: Ensure you are not exceeding sending limits or triggering spam filters.
- Examine Firewalls: Investigate firewall settings to confirm they are not blocking SMTP traffic.
- Contact Support: If the error persists, contact your email provider's support or the recipient's IT team.
- Mailop: If you are blocked, consider contacting Mailop to resolve the issue.
What email marketers say
10 marketer opinions
The SMTP deferred message 'refused to talk to me: 421 4.7.0 Not allowed' generally indicates a temporary issue preventing email delivery. Common causes include IP-based blocking, server configuration problems (DNS, SPF, DKIM), exceeding sending limits, greylisting, firewall interference, rate limiting by the recipient server, or the recipient server being temporarily unavailable. Resolution often involves checking server configurations, contacting support (e.g., GoDaddy), or retrying sending the email later.
Key opinions
- IP Blocking: The recipient server may have blocked your IP address or domain. Check blacklists.
- Server Configuration: Issues with DNS records (MX, SPF, DKIM) or reverse DNS (rDNS) can cause this error.
- Sending Limits: Exceeding sending limits or triggering spam filters can result in a 421 error.
- Greylisting: The recipient server might be using greylisting, temporarily rejecting emails from unknown senders.
- Firewall Interference: A firewall might be blocking the SMTP server's connection.
- Rate Limiting: The recipient server might be limiting the number of emails accepted from your server.
- Server Unavailability: The recipient's mail server might be temporarily busy or unavailable.
Key considerations
- Check Blacklists: Verify your IP address or domain is not on any email blacklists.
- Review Configuration: Ensure your DNS records, SPF records, DKIM records, and reverse DNS are correctly configured.
- Adjust Sending Volume: If rate limiting is suspected, reduce the volume of emails being sent within a given timeframe.
- Retry Sending: As the error often indicates a temporary issue, retry sending the email after a short period.
- Contact Support: If the problem persists, contact your email hosting provider or the recipient's support team.
- Firewall Settings: Review your firewall settings to ensure they are not interfering with email traffic.
Marketer view
Email marketer from Digital Ocean mentions that 421 errors often appear when there are issues with the configuration of your mail server, such as incorrect DNS settings or missing authentication records. They suggest reviewing your setup to ensure it meets best practices.
17 Nov 2021 - Digital Ocean
Marketer view
Marketer from Email Geeks shares that the SMTP deferred message likely indicates a manual IP-based block and suggests contacting the provider or using Mailop.
2 Apr 2025 - Email Geeks
What the experts say
2 expert opinions
A '421 4.7.0 Not allowed' SMTP error indicates a temporary problem with the receiving mail server. It may be due to greylisting, server overload, maintenance, or anti-spam measures. Retrying the email is essential, but persistent errors suggest a deeper configuration issue requiring investigation.
Key opinions
- Greylisting: Receiving servers might use greylisting, temporarily rejecting emails from unknown senders to combat spam.
- Temporary Server Issues: The error can stem from server overload, scheduled maintenance, or other temporary anti-spam measures.
Key considerations
- Retry Sending: Always retry sending the email after a short delay, as the issue might resolve itself.
- Investigate Configuration: If the error persists, investigate potential underlying configuration issues in your email setup.
Expert view
Expert from Spamresource explains that a 421 error can occur due to greylisting techniques employed by receiving mail servers as a method to combat spam. The server temporarily rejects emails from unknown senders, with the expectation that legitimate senders will retry sending the email later.
13 Apr 2024 - Spamresource
Expert view
Expert from Word to the Wise explains that a 421 SMTP error can indicate several temporary server issues, including server overload, maintenance, or anti-spam measures. It emphasizes that retry attempts are crucial, but persistent 421 errors may suggest underlying configuration problems that need investigation.
20 Jan 2024 - Word to the Wise
What the documentation says
4 technical articles
The '421 4.7.0 Not allowed' SMTP error indicates a temporary service unavailability. The sending server should retry later, as the issue is transient and may be due to server overload, maintenance, or temporary blocking policies. The error signifies the server knows it cannot provide the service at the moment but anticipates a later retry might succeed.
Key findings
- Service Unavailable: The 421 error means the service is temporarily unavailable.
- Transient Issue: The issue is transient and the sender should retry.
- Possible Causes: Causes can include server overload, maintenance, or temporary blocking policies.
Key considerations
- Retry Later: The sending server should retry sending the email later.
Technical article
Documentation from cPanel Documentation explains that a 421 SMTP error signifies that the service is temporarily unavailable and suggests that the sending server should try again later. It could be due to server overload, maintenance, or temporary blocking policies.
20 Feb 2022 - cPanel Documentation
Technical article
Documentation from Microsoft Docs explains that a 421 error, including variations like '4.7.0 Not allowed', generally signifies 'Service not available, closing transmission channel'. This can indicate a temporary issue on the server side, such as being overloaded or undergoing maintenance.
20 Sep 2021 - Microsoft Docs
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