How to resolve a 'connection refused' network error when sending emails?
Michael Ko
Co-founder & CEO, Suped
Published 17 Jun 2025
Updated 17 Aug 2025
9 min read
Encountering a 'connection refused' error when attempting to send emails can be frustrating. This message indicates that your email server, or the application trying to send the email, was unable to establish a connection with the recipient's mail server. Unlike a bounce message that specifies a reason like '550 relaying denied' or an IP address blacklist, 'connection refused' points to a network or server availability issue rather than a content or policy violation. It effectively means the door was shut before a conversation could even begin.
This error is common across various network applications, not just email, and implies that the target server actively rejected the connection attempt, or there was no service listening on the specified port. It's distinct from a connection timeout error, where the connection simply never establishes within a given timeframe.
To effectively resolve a 'connection refused' error, you need to systematically diagnose the underlying cause. This guide will walk you through the most common scenarios and provide actionable steps to get your emails flowing again.
Understanding the 'connection refused' error
The 'connection refused' error, also known as `ECONNREFUSED` on Linux or `WSAECONNREFUSED` on Windows, primarily indicates that the client attempted to connect to a server that actively refused the connection. This isn't a passive failure, but an active denial. It means that the server's operating system explicitly denied the connection request, often because no service was listening on the requested port or due to firewall rules. Unlike a relaying denied message, which comes from the SMTP server itself, 'connection refused' is a lower-level network issue.
For email, this typically means your mail server couldn't establish a TCP connection to the recipient's mail exchanger (MX) server on the standard SMTP port (usually 25, 587, or 465). This can stem from various points in the network path, from the sending server's local configuration to issues on the receiving end, or even intermediate network devices. Understanding where the refusal originates is key to diagnosing the problem.
It is crucial to distinguish this from issues where the email is rejected after initial connection, such as spam filtering or sender policy errors. The 'connection refused' error prevents even the first handshake, signaling a fundamental barrier in communication. For more general insights into why this error occurs across different applications, you can review common reasons for connection refused errors.
Common causes: Firewall and network issues
Firewalls are a frequent culprit for 'connection refused' errors. Both the sender's firewall and the recipient's firewall can block outbound or inbound connections on specific ports, leading to this refusal. Your server's firewall might be preventing outgoing connections to the SMTP port, or the recipient's mail server firewall could be configured to block connections from your IP address or network range. This often happens if your IP has been flagged for suspicious activity, even if it's not on a public blocklist (blacklist).
Network routing issues can also cause this problem. If your server cannot find a valid route to the recipient's mail server, or if there's an intermediate router or network device actively dropping or refusing packets destined for the mail server, you will see this error. This can be complex to diagnose without access to network diagnostics tools like traceroute. Invalid DNS resolution for the recipient's MX record can also lead to attempts to connect to the wrong, non-existent, or unreachable IP address, resulting in a 'connection refused' message.
Sometimes, the issue is as simple as incorrect proxy settings on your sending server or within your network. If your system is configured to use a proxy that isn't properly set up or is blocking the necessary ports, it can intercept and refuse the connection. VPNs can also sometimes interfere with direct mail server connections, leading to similar refusal errors. Checking these basic network components first can save significant troubleshooting time.
Common causes: Server and configuration issues
On the recipient's side, a 'connection refused' error often means there's no mail server process (like Postfix, Exchange, or Sendmail) actively listening on the standard SMTP port. This could be due to the mail server being down, crashing, or simply not being configured to listen on the port your sending server is attempting to connect to. Even if the server is running, an incorrect port configuration can cause the refusal. Ensure you are using the correct SMTP port for the recipient's server; common ports are 25 for unencrypted, 587 for Submission (STARTTLS), and 465 for SMTPS (SSL/TLS).
Another server-side issue can be the mail server being overloaded or experiencing temporary resource exhaustion. While less common for a full 'connection refused' and more typical for a temporary error (like a 421 code), a severely overloaded server might actively drop new connection attempts to prevent further degradation. Similarly, if the mail server has a strict connection limit and is already at capacity, it may refuse new connections.
Less frequently, but still possible, are issues with the recipient's SMTP endpoint itself. For instance, if you are sending to a large provider, their specific configuration might lead to a refusal if certain conditions aren't met, such as attempting to connect from a disallowed region or via a deprecated method. These cases often require checking the provider's specific postmaster guidelines or troubleshooting documentation.
Advanced troubleshooting steps
To pinpoint the exact cause, start with basic network diagnostics from your sending server. The telnet command is invaluable for this. Try to telnet to the recipient's MX server on the appropriate port. If it connects, the problem isn't a fundamental network blockage. If it refuses, you've confirmed a network or server-side listen issue.
Telnet command exampleBASH
telnet mail.example.com 25
Next, use ping and traceroute to test connectivity to the recipient's MX server IP address. Ping verifies basic IP reachability, while traceroute maps the network path, helping identify if the connection is refused at an intermediate hop, indicating a routing issue or an ISP-level block. Check your own server's firewall rules (e.g., iptables, ufw, Windows Firewall) to ensure outbound SMTP traffic isn't being blocked. Also, verify that your email sending application is configured with the correct SMTP host and port.
If all local and immediate network checks pass, the issue likely resides on the recipient's side. At this point, you may need to contact the recipient's IT department or mail administrator. Provide them with the exact error message, your sending IP address, and the target IP address and port, as well as the results of your telnet and traceroute tests. They can then check their firewalls, mail server status, and configurations, including whether your IP is on their internal blocklist (blacklist).
Prevention and monitoring
Preventing 'connection refused' errors largely comes down to proactive monitoring and proper configuration. Regularly audit your sending server's firewall rules to ensure they align with your email sending needs. If you rely on a third-party email service provider (ESP), ensure their network is stable and their IP addresses are not known for blocklisting (blacklisting) or connectivity issues with major mailbox providers.
Maintaining a healthy sender reputation can also indirectly help, as some receiving servers might be more lenient with connections from trusted IPs. While a direct 'connection refused' isn't usually a reputation issue, consistent good sending practices reduce the likelihood of your IP ending up on internal blocklists (blacklists) that could lead to such rejections. Implementing email authentication like DMARC, SPF, and DKIM can contribute to this trust.
For ongoing monitoring, consider tools that can continuously check the reachability of critical mail servers or alert you to changes in network routes. Regular deliverability testing can also proactively identify if your mail is being refused before it impacts a large number of recipients. Staying vigilant about your network health and server configurations is paramount.
Views from the trenches
Best practices
Regularly review and update your server's firewall rules to ensure outbound SMTP ports (25, 587, 465) are open and not inadvertently blocking connections.
Verify the recipient's MX records and target SMTP port before attempting to send to confirm you're connecting to the correct server and service.
Utilize an email service provider (ESP) with a robust infrastructure to offload complex network management and ensure higher deliverability rates.
Implement email authentication protocols like SPF, DKIM, and DMARC to build trust and reduce the likelihood of your IP being internally blocklisted.
Proactively monitor your email logs for 'connection refused' errors and other network-related issues to identify and address problems early.
Common pitfalls
Assuming a 'connection refused' error is an email blocklist issue immediately, without first verifying fundamental network connectivity.
Overlooking local network configurations, such as proxy settings or VPNs, which might be interfering with outgoing SMTP connections.
Neglecting to check the status or configuration of the recipient's mail server, which might be offline or not listening on the expected port.
Failing to use diagnostic tools like telnet, ping, and traceroute to systematically identify the point of connection failure.
Not communicating effectively with the recipient's IT team, providing insufficient details for them to troubleshoot issues on their end.
Expert tips
Always perform a telnet test to the target MX server on the relevant SMTP port to quickly determine if the issue is a network block or server availability.
If telnet fails, a traceroute can reveal if the refusal occurs at your local network, your ISP, or at an intermediate hop on the way to the destination.
Be mindful of transient network issues; sometimes a brief 'connection refused' can resolve itself, but persistent errors require deeper investigation.
Double-check your DNS resolvers to ensure they are correctly resolving MX records, as misconfigurations can lead to attempts to connect to incorrect IPs.
When contacting the recipient's administrator, provide your sending IP, the target IP and port, and any diagnostic output you've gathered.
Marketer view
A marketer from Email Geeks says: A 'connection refused' error often indicates a network issue, not necessarily an SMTP error or an email blocklist problem.
Feb 15, 2024 - Email Geeks
Marketer view
A marketer from Email Geeks says: To diagnose a connection refused error, first check if you can manually connect to the recipient's host on port 25 using tools like telnet or ping.
Feb 16, 2024 - Email Geeks
Summary of resolution
The 'connection refused' error, while disruptive, is a clear signal that the initial handshake between your server and the recipient's server failed. By systematically checking common culprits like firewalls, network routing, DNS, and server availability, you can usually identify and resolve the issue. Proactive monitoring and adherence to best practices in email deliverability will also help minimize the occurrence of such errors, ensuring your emails reach their intended destination.
