What causes the bounce code 4.7.0 'Too many concurrent connections' and how can it be resolved?
Matthew Whittaker
Co-founder & CTO, Suped
Published 28 Apr 2025
Updated 17 Aug 2025
9 min read
Encountering the 4.7.0 Too many concurrent connections bounce code can be frustrating for any email sender. It signals that the recipient's mail server is temporarily overwhelmed or has imposed a limit on the number of simultaneous connections it will accept from your sending IP address. While often a temporary issue, persistent occurrences can severely impact your email deliverability and communication with your recipients. It essentially means the receiving server is telling your mail server to slow down and try again later.
This error isn't necessarily about your email content being flagged as spam, but rather a technical bottleneck. It's akin to trying to fit too many cars onto a narrow bridge at once. The server simply can't handle the volume of incoming connections at that specific moment. Understanding the nuances of this bounce code is crucial for diagnosing and resolving the underlying issues, which can range from your sending practices to the recipient server's configuration or even the reputation of your shared IP pool.
The good news is that 4.7.0 is a soft bounce, meaning the mail server is asking your system to retry later. Most reputable email service providers (ESPs) have built-in retry mechanisms for these types of transient errors, but if the issue persists, messages might eventually be discarded. This guide will delve into what causes these errors and provide actionable strategies to mitigate them and ensure your emails reach their intended destinations.
Why mail servers have connection limits
Mail servers, particularly those operated by larger internet service providers (ISPs) and mailbox providers, implement connection limits for several reasons. Primarily, these limits are in place to prevent resource exhaustion and to maintain system stability. Each open SMTP connection consumes server resources, and an uncontrolled influx of connections could easily lead to server slowdowns or crashes. By limiting concurrent connections, servers can manage their load efficiently and ensure consistent service for all users.
Beyond resource management, connection limits serve as a crucial spam prevention mechanism. Spammers often attempt to send large volumes of unsolicited email by opening numerous simultaneous connections to mail servers. By imposing limits, receiving servers can detect and deter such abusive behavior, forcing spammers to slow down or switch tactics. This measure helps protect their users from unwanted emails and maintains the integrity of their network.
The specific limits can vary widely between different mail servers and ISPs. Some may have fixed, hard-coded limits, while others might dynamically adjust limits based on the sender's IP reputation. For instance, a sender with a strong, positive reputation might be allowed more concurrent connections than one with a questionable or unknown reputation. This dynamic approach makes it challenging to pinpoint an exact universal limit, necessitating a proactive approach to managing your sending practices.
Common causes of 'too many concurrent connections'
The primary cause of the 4.7.0 Too many concurrent connections error is sending too many emails too quickly from a single IP address to a specific receiving server. This often happens when a sudden burst of emails overwhelms the recipient server's capacity or triggers its rate-limiting defenses. It's a clear signal that the receiving end is unable to process the volume of connections your server is attempting to establish at that moment.
Shared IP addresses are a common culprit, especially for senders using an email service provider. On a shared IP, your email traffic is mixed with that of other users. If another user on the same IP is sending high volumes or engaging in questionable sending practices, it can negatively impact the IP's reputation. This degraded reputation might lead recipient servers to impose stricter connection limits on the entire shared IP, affecting your legitimate emails as well.Broadcom's support page on this error highlights that the destination mail server may be rate limiting delivery.
Another contributing factor can be poor sender reputation. Even if your sending volume isn't exceptionally high, a low or fluctuating sender reputation can cause recipient servers to be more cautious and quickly enforce connection limits. This is because a poor reputation often indicates a higher likelihood of spam or unwanted mail, prompting stricter filtering policies. Therefore, maintaining a healthy sender reputation is paramount to avoiding these types of errors, as well as mitigating the risk of your IP or domain ending up on a blocklist (or blacklist).
Typical causes
The bounce code 4.7.0 can be tricky because it might not provide a direct root cause, but common factors include a sudden surge in sending volume, issues with shared IP pools, or a receiving server that's temporarily overloaded or configured with very strict limits.
Strategies for resolving connection limits
When facing 4.7.0 bounce errors, the first step is to assess your sending volume and speed. If you're sending a large batch of emails, consider segmenting your campaigns or implementing a slower sending rate. Many ESPs allow you to adjust these parameters. For instance, Mimecast's documentation mentions exceeding outbound thread limits as a cause and suggests sending messages in smaller chunks.
If you're using a shared IP, and especially if your transactional emails are experiencing these issues, it's worth discussing this with your ESP. They might be able to adjust connection limits on their end or investigate the quality of the shared IP pool. In some cases, a transition to a dedicated IP address might be the most effective long-term solution. A dedicated IP gives you more control over your sending reputation and reduces the risk of being impacted by other senders' issues. For specific examples, understanding how Hotmail handles connection errors or Optimum/Optonline.net limits can provide valuable insight.
Improving your overall sender reputation can also alleviate these issues. This involves consistently sending relevant and engaging content, maintaining a clean mailing list, and ensuring proper email authentication (SPF, DKIM, and DMARC). A strong reputation signals to recipient servers that your emails are legitimate and trustworthy, making them more likely to accept higher volumes of connections without imposing restrictions. Regularly checking your Sender Score and other reputation metrics can help you stay on top of your sending health.
Shared IP solutions
Contact ESP: Inquire about the shared IP pool's reputation and ask if they can adjust connection limits on their end. They may be able to re-route your traffic.
Consider Migration: Moving to a dedicated IP gives you full control over your sending volume and reputation, isolating you from other senders' issues.
IP Warm-up: If you switch to a dedicated IP, implement a gradual IP warm-up schedule to build trust with ISPs and avoid initial connection issues.
Advanced troubleshooting and prevention
Proactive monitoring of your email deliverability metrics is key to preventing 4.7.0 errors. Keep a close watch on your bounce logs, specifically filtering for temporary failures related to concurrent connections. Tools that provide real-time insights into your sending reputation and inbox placement can help you identify potential problems before they escalate. Consistent monitoring allows for immediate adjustments to your sending strategy.
Engaging in consistent email sending practices, often referred to as maintaining a steady sending cadence, is a crucial part of building and maintaining a positive sender reputation. Avoid large, infrequent blasts that can appear suspicious to mail servers. Instead, distribute your sending volume more evenly over time. This helps ISPs learn to trust your sending patterns and makes them less likely to impose sudden connection limits. This also contributes to overall email deliverability.
For ongoing issues, especially with specific mailbox providers, establishing a direct relationship with their postmaster teams can be beneficial. Many ISPs have postmaster pages where you can report issues or request delisting from internal blacklists (blocklists). This direct communication can sometimes lead to faster resolution and a better understanding of their specific sending policies. While Google Postmaster Tools offer valuable insights, direct communication can sometimes fill in the gaps for persistent problems.
Views from the trenches
Best practices
Monitor your bounce logs regularly for a 4.7.0 error pattern and track affected domains.
Segment large email sends into smaller batches to reduce the instantaneous connection demand.
Maintain a consistent sending volume and schedule to build a predictable reputation.
Ensure proper email authentication (SPF, DKIM, DMARC) to bolster your sender trustworthiness.
Common pitfalls
Ignoring 4.7.0 bounces as merely temporary failures, leading to increasing discard rates.
Underestimating the impact of shared IP pool quality on connection limits and deliverability.
Failing to communicate with your email service provider about persistent connection issues.
Not adjusting sending rates to accommodate the varying connection limits of different ISPs.
Expert tips
Actively engage with your email service provider to discuss specific connection limit adjustments for critical recipients.
Implement intelligent retry logic in your sending system that respects recipient server limits.
Analyze bounce messages for specific ISP names to identify targeted adjustments or communication needs.
Consider a dedicated IP address if shared IP performance consistently triggers connection errors.
Expert view
Expert from Email Geeks says: Mailbox providers (MBPs) might have fixed limits or dynamic limits based on sender reputation, and the sending service provider is usually responsible for configuring these.
2024-10-08 - Email Geeks
Expert view
Expert from Email Geeks says: Unless a sender can directly configure connection limits, the sending service provider is responsible for managing these settings.
2024-10-08 - Email Geeks
Ensuring uninterrupted email flow
Successfully managing 4.7.0 Too many concurrent connections bounce errors requires a combination of technical adjustments and strategic sending practices. It's not just about overcoming a temporary hurdle, but about building a robust and reliable email program that respects the limitations and preferences of recipient servers. By actively monitoring your sending health and proactively addressing potential issues, you can minimize these bounces and improve your overall email deliverability.
Remember, this bounce code is a signal to optimize your sending behavior. Whether it's adjusting your sending rate, working closely with your ESP, or considering a dedicated IP address, each step contributes to a healthier email ecosystem. Persistent temporary failures can eventually lead to permanent blocking, making early intervention critical. A comprehensive email deliverability test can help uncover these and other issues.
Google Postmaster Tools offer valuable insights, direct communication can sometimes fill in the gaps for persistent problems.
