What does SMTP deferred message 'refused to talk to me: 421 4.7.0 Not allowed' mean?
Matthew Whittaker
Co-founder & CTO, Suped
Published 28 Jun 2025
Updated 18 Aug 2025
8 min read
When you send an email and receive an SMTP deferred message like "refused to talk to me: 421 4.7.0 Not allowed," it means the recipient's mail server temporarily refused the connection. This is a soft bounce, indicating that the mail server isn't outright rejecting your message but is currently unwilling to accept it due to a policy or transient issue. While these errors are typically temporary, they signal underlying problems that need attention to ensure your emails reach their intended inboxes.
The message implies that the receiving server (for example, mx.online.no) has detected something it deems suspicious or a policy violation, prompting it to defer delivery. This isn't a permanent failure, but rather a request to "try again later" after addressing the perceived issue. However, if the underlying problem isn't resolved, these deferrals can lead to permanent rejections or impact your sender reputation over time.
Understanding the specific meaning behind each part of the error code can help you diagnose the root cause and implement appropriate solutions.
Understanding the 421 4.7.0 error code
The "421" SMTP response code is a temporary error, indicating that the service is unavailable, but the sending mail server should try again later. It signals that the receiving server is experiencing a transient issue or is temporarily unwilling to accept mail. Unlike 5xx errors, which are permanent rejections, a 4xx error means there's still a chance for delivery upon retry.
The "4.7.0" status code provides more specific context. In the SMTP extended status codes, "4.7.0" typically signifies a security or policy violation. This could relate to issues such as suspicious activity detected from your sending IP, rate limiting, or even a general server-side policy that prohibits connections based on certain criteria. The additional phrase "Not allowed" further reinforces that the refusal is due to a policy on the receiving server's end.
This deferral can stem from various reasons, from too many concurrent connections, as mentioned by Broadcom, to detected unusual sending rates, as described in a Google support thread. The key is that the issue isn't with the recipient address itself but with the connection parameters or the perceived reputation of the sender.
Example SMTP Deferred Message
421 4.7.0 Not allowed
Understanding 421 errors
A 421 SMTP error is a temporary deferral by the receiving mail server. It signifies that the server is currently unable or unwilling to accept your email due to policy restrictions or resource limitations. While your mail server will typically retry sending, repeated 421 errors can negatively impact your sender reputation and lead to long-term deliverability issues.
Common causes of this deferred message
The "421 4.7.0 Not allowed" message can arise from several common scenarios related to how the receiving server perceives your sending activity or its own operational status. Identifying the specific cause is crucial for a targeted resolution.
Rate limiting or too many connections: Many mail servers implement limits on the number of emails or connections allowed from a single IP address within a specific timeframe. If you exceed these thresholds, the server might temporarily defer your mail to prevent abuse or overload. This is a common reason for 421 errors.
IP or domain reputation issues: If your sending IP address or domain has a poor reputation, or if it appears on a public or private blacklist (or blocklist), the receiving server might temporarily refuse connections as a spam prevention measure. They're basically saying, "We don't trust you enough to talk to you right now."
Authentication failures: While less common for a 421, weak or misconfigured email authentication records (SPF, DKIM, DMARC) can contribute to a server's distrust, leading to deferrals. If your email cannot be verified as legitimate, it may be temporarily blocked.
Recipient server issues: Sometimes the issue isn't with your sending, but with the receiving server itself. It might be experiencing high load, undergoing maintenance, or having network problems, causing it to temporarily reject new connections.
Understanding these potential causes is the first step in effectively troubleshooting and resolving the deferred message.
Reputation-based deferrals
Spam complaints: High complaint rates from previous sends can quickly degrade your sender reputation.
Blocklist listing: Your IP or domain might be listed on a Real-time Blackhole List (RBL) or other anti-spam email blocklist.
Sending volume spikes: Sudden increases in email volume can trigger spam filters and rate limits.
Technical or policy-based deferrals
Concurrent connections exceeded: The receiving server may have a limit on the number of simultaneous connections.
Greylisting: A temporary rejection used to thwart spam, requiring the sending server to retry after a delay.
Misconfigured DNS: Invalid or missing DNS records (e.g., reverse DNS) can lead to distrust.
Diagnosing and troubleshooting the error
Once you encounter a "421 4.7.0 Not allowed" deferred message, acting quickly is essential to prevent further disruption to your email deliverability. Here's a systematic approach to diagnosing and troubleshooting the issue.
First, review your mail server logs. Look for patterns: Is the error occurring with a specific recipient domain, or is it widespread? Are there particular times of day when these errors are more frequent? This can help you narrow down whether it's a specific domain's policy, a volume issue, or a general reputation problem. Also, examine the full bounce message for any additional context beyond the "421 4.7.0 Not allowed" part, as sometimes more specific reasons are provided.
Next, assess your sender reputation. Check if your sending IP address or domain is listed on any common email blacklists (or blocklists). Many large mail providers like Yahoo use these lists to filter incoming mail. Ensure your email authentication (SPF, DKIM, and DMARC) is correctly configured and aligned, as this plays a significant role in establishing trust with recipient servers. A strong authentication setup is fundamental for consistent deliverability.
If you suspect rate limiting, consider temporarily reducing your sending volume to the affected domain and gradually increasing it. If the issue persists and you've ruled out common problems on your end, reaching out to the recipient domain's postmaster or abuse department is the next step. While it can be challenging to get a response, as noted in the Slack thread, persistent efforts or seeking advice from mail operation communities like Mailop can be helpful.
Issue
Check
Action
IP/Domain Reputation
Check your IP and domain on common email blocklists/blacklists.
If listed, request delisting and address root causes like spam complaints.
Sending Volume
Review if recent sending volume to the affected domain has increased rapidly.
Gradually reduce and then increase your sending rate to avoid triggering rate limits.
Email Authentication
Verify your SPF, DKIM, and DMARC records for correctness and alignment using a DMARC record generator.
Correct any misconfigurations to improve trust with receiving servers.
Recipient Server Policy
Check if the issue is isolated to a specific domain or affects multiple recipients.
Contact the recipient's postmaster if other checks yield no issues on your side.
Preventing future 421 4.7.0 errors
Preventing "421 4.7.0 Not allowed" errors requires a proactive approach to email deliverability, focusing on maintaining a strong sender reputation and adhering to best practices.
One crucial step is to consistently monitor your email metrics, including bounce rates, spam complaint rates, and inbox placement. Tools like Google Postmaster Tools can provide valuable insights into your domain and IP reputation. Regularly cleaning your mailing lists to remove inactive or invalid addresses will also help reduce bounces and spam trap hits, which can negatively impact your sender score.
Ensure your SPF, DKIM, and DMARC records are properly configured and maintained. These authentication protocols help receiving servers verify that your emails are legitimate and sent from authorized sources, significantly boosting their trust in your mail. For new sending IPs or domains, a gradual email warm-up process is vital to build a positive sending history with Internet Service Providers (ISPs), preventing them from flagging sudden, high-volume sends as suspicious.
List hygiene: Regularly clean your subscriber lists to remove invalid or unengaged addresses.
Email authentication: Implement and maintain strong SPF, DKIM, and DMARC policies.
Volume control: Avoid sudden spikes in sending volume, especially to new recipient domains.
Key takeaways
The SMTP deferred message "refused to talk to me: 421 4.7.0 Not allowed" is a temporary setback, but it's a clear signal from the receiving server that something is amiss. It could be due to exceeding rate limits, being on a blocklist (or blacklist), or simply appearing suspicious to their anti-spam filters. Ignoring these warnings can lead to more severe deliverability issues down the line, including permanent rejections and damage to your sender reputation.
By understanding the nuances of this error, diligently checking your logs and reputation, ensuring proper authentication, and employing proactive strategies, you can resolve these deferrals and maintain a healthy email sending infrastructure. Consistent monitoring and adherence to email best practices are key to ensuring your messages reliably reach the inbox.
Views from the trenches
Best practices
Monitor your sending reputation consistently across various providers to catch issues early.
Implement a gradual warm-up strategy for new IPs or domains before sending large volumes.
Keep your email lists clean by regularly removing bounced and unengaged addresses.
Common pitfalls
Ignoring 4xx temporary errors, assuming they always resolve themselves without intervention.
Making sudden, large increases in sending volume without proper IP warm-up.
Failing to check for IP or domain listings on common blacklists or blocklists.
Expert tips
Regularly check your IP's reverse DNS (rDNS) to ensure it correctly resolves to your sending domain.
If reaching the postmaster is difficult, try alternative communication channels like their public forums or social media, if available.
Consider using a dedicated IP address for your email sending if volume is high to better control your reputation.
Expert view
Expert from Email Geeks says manual IP-based blocks can be a cause, and often there isn't enough information initially to appeal or self-resolve effectively.
July 9, 2021 - Email Geeks
Marketer view
Marketer from Email Geeks says it can be challenging to contact the receiving domain, especially if support is only for paying customers or general abuse/postmaster emails go unanswered.
