How to resolve a 'connection refused' network error when sending emails?

Summary

A 'connection refused' error when sending emails is often due to network issues or a misconfigured mail server. Initial steps involve verifying the recipient's IP address and port, ensuring the mail service is running, and checking firewalls on both sides. If network connectivity is the issue, traceroute and manual connection tests on port 25 are recommended. The SMTP banner should be checked for explicit error messages. If the network is working, greylisting, blacklisting, server overload, incorrect DNS settings, and authentication failures should be considered. Debugging tools for Postfix, Exim, and Sendmail can help identify the specific cause. External tools like MXToolbox and contacting support teams may also be necessary.

Key findings

  • Network Connectivity: The sender's server may not be able to connect to the receiving server due to network issues.
  • Service Status: The receiving mail service might not be running or listening on the correct port.
  • Firewall Issues: Firewalls on the sending or receiving server might be blocking the connection.
  • DNS Problems: Incorrect DNS settings can prevent the sender's server from finding the recipient's server.
  • Blacklisting: The sending IP address may be blacklisted.
  • Greylisting: The receiving server may be temporarily refusing connections from unknown senders (greylisting).
  • SMTP Banner Information: The SMTP banner may contain specific error messages regarding the refusal.

Key considerations

  • Traceroute/Port Check: Use traceroute and manual port 25 connection tests to check network connectivity.
  • Contact Support: Consider contacting support for Synaq or your email service provider.
  • Debugging Tools: Utilize debugging options for Postfix, Exim, or Sendmail to identify specific errors.
  • MXToolbox: Use MXToolbox to check for DNS issues and blacklist status.
  • Check Server Load: Ensure the server isn't overloaded or has resource limits.
  • Retry Sending: Retry sending the email after a short delay, in case of greylisting.

What email marketers say
9Marketer opinions

A 'connection refused' network error when sending emails can stem from various issues on both the sending and receiving ends. Solutions involve checking if the receiving mail service is running and listening on the correct port, verifying firewall configurations on both sides to ensure SMTP ports (25, 465, 587) are open, and ensuring correct DNS resolution. Blacklist status should be checked, as should the possibility of greylisting. The SMTP banner should be examined, and it is necessary to test basic network connectivity with tools like `telnet` or `nc`. Overloaded servers and misconfigured mail server software can also cause this error. Contacting support or using online tools for diagnosis is often advised.

Key opinions

  • Service Status: Ensure the receiving mail service is running and listening on the correct port. Use tools like `netstat` or `ss` to verify.
  • Firewall Configuration: Verify firewall settings on both the sending and receiving servers to allow connections on SMTP ports (25, 465, 587).
  • DNS Resolution: Confirm that the domain name resolves to the correct IP address.
  • Blacklist Status: Check if your sending IP address is blacklisted.
  • Network Connectivity: Test basic network connectivity with tools like `telnet` or `nc`.
  • SMTP Banner: Examine the SMTP banner for specific error messages.

Key considerations

  • Greylisting: Be aware of greylisting and retry sending the email after a short delay.
  • Server Overload: Consider that the receiving mail server might be overloaded or temporarily unavailable; try again later.
  • Mail Server Configuration: Ensure your mail server software (e.g., Postfix, Sendmail) is properly configured.
  • Support Contact: Contact support or use online tools for diagnosis if basic troubleshooting steps don't resolve the issue.
  • Synaq Contact: Consider contacting synaq.com if they are being used as the MX gateway.
Marketer view

Email marketer from SiteGround explains checking the server's status page to see if there are any known issues affecting mail services. If there are no known issues, contact their support team for further assistance.

November 2024 - SiteGround
Marketer view

Email marketer from MXToolbox explains using their tools to check for DNS issues and blacklist status. Incorrect DNS settings can prevent your mail server from finding the recipient's server, and being on a blacklist will cause connections to be refused.

August 2021 - MXToolbox
Marketer view

Marketer from Email Geeks suggests contacting someone at synaq.com and offers to organize a tracert.

October 2022 - Email Geeks
Marketer view

Email marketer from StackOverflow recommends first checking if the service is running on the target machine and listening on the expected port. They advise using tools like `netstat` or `ss` to verify this. Next, they suggest checking firewall settings that might be blocking the connection. Finally, they propose testing connectivity with `telnet` or `nc` to isolate the issue.

January 2024 - StackOverflow
Marketer view

Email marketer from Email on Acid shares using telnet to test your SMTP connection. If you can't connect with telnet, the problem likely lies with your network or firewall.

June 2023 - Email on Acid
Marketer view

Email marketer from SuperUser forum says that a common reason is the mail server being overloaded or temporarily unavailable. They recommend trying again later. Also, ensure your sending IP address isn't blacklisted by the receiving mail server.

January 2022 - SuperUser
Marketer view

Email marketer from cPanel forums recommends carefully configuring the firewall to allow outgoing connections on the necessary SMTP ports (25, 465, 587). Also, ensure that the firewall isn't blocking incoming connections to your own mail server.

October 2024 - cPanel Forums
Marketer view

Email marketer from DigitalOcean shares making sure that your mail server software (e.g., Postfix, Sendmail) is properly configured and listening on the correct port. It also means checking your server's resource usage (CPU, memory) to ensure it's not overloaded.

April 2023 - DigitalOcean
Marketer view

Email marketer from Reddit shares that common causes include the service not running, a firewall blocking the connection, or incorrect DNS resolution. Suggests verifying that the mail server is running, that the firewall allows connections on port 25, 465, or 587, and that the domain name resolves to the correct IP address.

July 2023 - Reddit

What the experts say
3Expert opinions

A 'connection refused' error when sending emails can be due to network issues preventing the sender's server from reaching the recipient's server. This might stem from routing problems, a deliberate block, or greylisting (temporary refusal). Examining the SMTP banner can offer clues about the reason for the refusal, such as connection limits or blacklisting. Retrying the email after a delay is recommended to address potential greylisting.

Key opinions

  • Network Error: The sender's server is unable to connect to the receiving server.
  • Routing Issues: Routing table problems might prevent the connection.
  • SMTP Banner: The SMTP banner may contain specific reasons for connection refusal.
  • Greylisting: Temporary refusal due to greylisting is a possibility.

Key considerations

  • Traceroute: Run a traceroute to identify routing problems.
  • Port 25 Check: Check connectivity manually on port 25.
  • Retry Sending: Retry sending the email after a short delay to address potential greylisting.
  • Synaq MX Gateway: Consider contacting Synaq if they are being used as the MX Gateway
Expert view

Expert from Spam Resource explains to check the SMTP banner for clues. The banner is what the receiving server says when you first connect to it. Sometimes, it will give a specific reason for refusing the connection, such as 'too many connections' or 'blacklisted IP'.

November 2023 - Spam Resource
Expert view

Expert from Word to the Wise explains greylisting, where the receiving server temporarily refuses the connection from an unknown sender. This is done to block spammers who don't retry sending. A legitimate server should retry after a delay. Therefore, retrying the email after a short period is the solution.

June 2024 - Word to the Wise
Expert view

Expert from Email Geeks explains that the error is a network error, meaning the sender's server cannot connect to the receiving server. It can't reach it due to a routing table issue. Suggests checking connectivity manually on port 25 and running a traceroute to the IP address to determine if it's a routing failure or a deliberate block. Mentions that they seem to be using synaq.com as their MX gateway.

March 2025 - Email Geeks

What the documentation says
5Technical articles

Troubleshooting a 'connection refused' error involves verifying the destination IP address and port, ensuring the target service is running and listening on the specified port, and checking firewall rules on both the client and server. Nmap can be used to scan the target host. Examining mail server logs (Postfix, Exim, Sendmail) and using debugging options provide further insights into the reason for the connection failure, which may include authentication failures, policy restrictions, or resource limits.

Key findings

  • Active Refusal: The target host is actively refusing the connection.
  • IP and Port: Incorrect IP address or port can cause connection refusal.
  • Service Status: The target service may not be running or listening on the specified port.
  • Firewall Rules: Firewall rules may be blocking the connection.
  • Port Status: Using Nmap reveals if the port is open, filtered, or closed.

Key considerations

  • Mail Server Logs: Check Postfix, Exim, or Sendmail logs for error messages.
  • Debugging Options: Use debugging options (e.g., `exim -d`, `sendmail -v`) to trace the connection attempt.
  • Authentication: Consider potential authentication failures.
  • Policy Restrictions: Consider policy restrictions on the sending server.
  • Resource Limits: Consider resource limits on the receiving server.
Technical article

Documentation from Postfix.org explains checking the Postfix logs for clues about the connection failure. It suggests looking for error messages that indicate why the connection was refused, such as authentication failures, policy restrictions, or resource limits.

June 2023 - Postfix.org
Technical article

Documentation from Exim.org explains using Exim's debugging options to trace the connection attempt and identify where the connection is being refused. This often involves using the `-d` flag with increasing levels of verbosity to see the SMTP conversation.

January 2024 - Exim.org
Technical article

Documentation from Nmap.org explains the using nmap can help diagnose network connectivity. It suggests using Nmap to scan the target host and port to check if the port is open, filtered, or closed. A 'closed' port often indicates that the connection is being refused. Firewalls or service unavailability can be the cause.

January 2023 - Nmap.org
Technical article

Documentation from Sendmail.org explains using `sendmail -v` to run Sendmail in verbose mode. This will provide more detailed output about the connection process, including any errors encountered.

April 2024 - Sendmail.org
Technical article

Documentation from Microsoft Learn explains that a 'connection refused' error indicates that the target host is actively refusing the connection attempt. It suggests verifying the destination IP address and port, ensuring the target service is running and listening on the specified port, and checking firewall rules on both the client and server to allow the connection.

August 2023 - Microsoft Learn