Summary
The bounce code 4.7.0, indicating 'Too many concurrent connections', typically arises from exceeding connection limits or being rate-limited by the receiving server, often as a protective measure against spam or DoS attacks. This issue can be exacerbated by the use of shared IPs. Solutions include contacting your ESP to adjust connection limits, transitioning to a dedicated IP, warming up new IPs, managing sending rates, reviewing server settings, ensuring a positive sender reputation, and analyzing server logs. The error signals a temporary delivery deferral, suggesting the email may be accepted later.
Key findings
- Cause: Exceeding connection limits or rate limiting by the receiving server.
- Shared IPs: Shared IP addresses increase the likelihood of encountering connection limits.
- Temporary: The 4.7.0 error indicates a temporary issue; delivery is deferred, not failed.
- Security: Connection limits protect against spam and denial-of-service attacks.
- ESP Role: ESPs may need to adjust connection limits on shared infrastructure.
Key considerations
- Contact ESP: Contact your Email Service Provider (ESP) to investigate and adjust connection limits.
- Dedicated IP: Consider switching to a dedicated IP address for greater control and fewer restrictions.
- IP Warm-up: Properly warm up new IPs to avoid triggering rate limits.
- Sending Rate: Manage sending rates to avoid overwhelming receiving servers.
- Server Settings: Review server settings to ensure compliance with receiving server limits.
- Reputation: Maintain a good sender reputation to minimize the chance of being throttled.
- Log Analysis: Analyze email headers and server logs to diagnose the specific cause of the error.
What email marketers say
13 marketer opinions
The 4.7.0 'Too many concurrent connections' bounce code arises primarily from exceeding connection limits or being rate-limited by receiving mail servers, often as an anti-spam measure. This can occur due to high email volumes, shared IP resources, or temporary server unavailability. Resolutions involve contacting the ESP for connection limit adjustments, migrating to a dedicated IP, warming up new IPs, spacing out email sends, reviewing server settings and logs, and ensuring a good sender reputation.
Key opinions
- Root Cause: Exceeding connection limits or rate-limiting by the receiving server.
- Shared IP Issues: Shared IPs are more prone to this error due to shared resource constraints.
- Temporary Nature: The error is often temporary, with emails eventually being delivered.
- High Volume Sending: Sending a high volume of emails in a short period can trigger this error.
- Delivery Deferred: 4.7.0 means 'Delivery temporarily deferred.' The sending server will try again later.
Key considerations
- Contact ESP: Contact your Email Service Provider (ESP) for connection limit adjustments.
- Dedicated IP: Consider moving to a dedicated IP address to avoid shared resource limitations.
- IP Warm-up: Properly warm up new IPs to avoid being flagged for high connection rates.
- Sending Practices: Space out email sends and adhere to sending best practices.
- Server Settings: Review your server settings to ensure you are within the receiving server's limits.
- Sender Reputation: Maintain a good sender reputation to avoid being blocked or rate-limited.
- Log Analysis: Review email headers and server logs to diagnose the root cause.
Marketer view
Email marketer from MailPoet explains that shared SMTP servers often have connection limits, leading to 4.7.0 errors. Switching to a dedicated SMTP server or using a different port may alleviate this.
23 Aug 2022 - MailPoet
Marketer view
Marketer from Email Geeks shares that too many connections could be due to either the receiving mail server having a fixed limit or making decisions based on reputation. Only MailGun can change the connection limit, and it is a temporary failure so will likely eventually deliver.
9 Jul 2022 - Email Geeks
What the experts say
4 expert opinions
The 4.7.0 error typically arises from the receiving mail server throttling connections or reaching concurrent connection limits. Shared IPs are a contributing factor and often the onus is on the ESP (like Mailgun) to configure connection limits. The error is a temporary 4xx class error, meaning the email might be accepted in the future.
Key opinions
- Throttling: Receiving mail servers often throttle connections to protect their infrastructure.
- Shared IPs: Shared IP addresses can contribute to hitting connection limits.
- ESP Responsibility: ESPs like Mailgun often need to adjust connection limits on their end.
- Temporary Error: 4xx errors signify a temporary issue; the email may be delivered later.
Key considerations
- Contact ESP: Engage with your ESP to check and adjust connection limits.
- Shared IP Impact: Be aware that shared IP reputation and limitations can trigger this error.
- Retry Delivery: Understand that the email will likely be retried automatically.
Expert view
Expert from Word to the Wise explains that 4xx errors means that there was a temporary error when sending the message. The message was not accepted at that time, but may be accepted in the future. This is different than a 5xx error, where you'd need to modify the email before resending it.
14 May 2022 - Word to the Wise
Expert view
Expert from Spam Resource explains that 4.7.0 errors are often due to throttling by the receiving mail server. This is a deliberate mechanism to limit the rate of incoming connections and messages from a particular sender or IP address to protect their infrastructure.
13 Mar 2025 - Spam Resource
What the documentation says
4 technical articles
The 4.7.0 bounce code is generally caused by the receiving server's inability to handle the volume of connections, often due to temporary busyness, rate limiting, or anti-spam measures. These limits are deliberately put in place to protect against denial of service and spam attacks. Managing the number of concurrent connections and adjusting sending rates can help to prevent these errors, especially in systems like Postfix.
Key findings
- Temporary Issue: The error often indicates a temporary problem with the receiving server.
- Rate Limiting: Rate limiting and anti-spam measures can trigger the 4.7.0 error.
- DoS Protection: Connection limits are in place to protect against denial of service attacks.
- Simultaneous Connections: Exceeding the allowed number of simultaneous connections can cause the error.
Key considerations
- Connection Management: Manage concurrent connections and sending rates to avoid triggering the error.
- Server Configuration: Adjust server settings (e.g., in Postfix) related to connection rates.
- Anti-Spam Policies: Understand that receiving servers have policies to protect against spam.
Technical article
Documentation from Microsoft Learn explains that the 4.7.0 error typically indicates a temporary issue with the receiving server, such as being too busy to handle the connection. It can also arise from rate limiting or other anti-spam measures.
19 May 2022 - Microsoft Learn
Technical article
Documentation from Cisco.com details that concurrent connection limits are put in place to protect against denial of service or SPAM attacks. These limits can cause 4.7.0 errors if they are reached.
28 Mar 2022 - Cisco.com
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