Summary
A 'connection refused' error in email delivery signifies an active rejection from the recipient's mail server, rather than a passive timeout. This indicates the remote host is explicitly declining the connection request, often due to a firewall blocking port 25, the recipient's mail service not running or listening correctly, or the sender's IP address being blacklisted or having a poor reputation. Troubleshooting involves systematically checking network connectivity, firewall rules on both ends, DNS records, and the operational status of the recipient's mail server.
Key findings
- Active Rejection: A 'connection refused' error indicates an active, explicit rejection by the remote server, typically via a TCP RST packet. This differs from a timeout, where no response is received, signifying the server or a firewall is actively declining the connection.
- Server Not Listening: A primary cause is that the recipient's mail server software (e.g., Postfix, Exim) is not running, is offline, overloaded, or is not configured to listen for connections on the correct port, commonly port 25.
- Firewall Blocks: Firewalls are a frequent culprit. This includes outbound firewalls on the sending server, inbound firewalls on the recipient's server (like CSF), or network-level blocks that prevent the connection request from reaching the mail service on port 25.
- IP Reputation & Blacklisting: The sender's IP address might be blacklisted, subject to rate-limiting, or have a poor sending reputation. Recipient servers often refuse connections from IPs flagged for suspicious activity or spam.
- Network & DNS Issues: Incorrect DNS records, particularly MX records pointing to a non-existent or wrong server, or underlying network connectivity problems between the sender and recipient servers, can lead to connection refusal.
Key considerations
- Determine Scope: Before troubleshooting, identify the extent of the 'connection refused' problem. Determine if it affects specific email addresses, particular IP ranges, or has been ongoing for a certain period to narrow down potential causes.
- Verify Network Connectivity: Use tools like telnet to test connectivity to the recipient's server on port 25. Check network paths and ensure correct port configurations. A successful telnet connection to the remote mail server on port 25 means the port is open and the service is listening, indicating the issue lies elsewhere.
- Inspect Firewall Rules: Review firewall configurations on both your sending server (outbound rules on port 25, e.g., iptables, UFW, or cloud security groups) and consider potential inbound blocks on the recipient's side. Firewalls are a very common cause of connection refusal.
- Confirm Recipient Server Status: Ensure the recipient's mail server service, whether Postfix, Exim, or another MTA, is actively running and correctly configured to listen on the necessary network interface and port, usually 25. The server might be offline, overloaded, or the mail service might not be listening.
- Check DNS & Blacklists: Verify the recipient's MX (Mail Exchange) DNS records are accurate and point to a legitimate, active mail server. Also, monitor your sending IP's reputation and check if it has been blacklisted, as some servers refuse connections from IPs with poor reputations.
- Engage Recipient IT: For severe or persistent 'connection refused' errors, especially when other troubleshooting steps yield no results, contacting the recipient's IT department is often the most direct path to resolution. Be prepared to explain why your mail should be accepted.
- Examine Server Logs: Consult your mail server logs (e.g., /var/log/exim_mainlog for Exim, or Postfix logs) for detailed error messages. These logs can provide specific reasons for the connection refusal from the remote server's perspective.
What email marketers say
7 marketer opinions
When an email delivery attempt results in a 'connection refused' error, it signals that the recipient's mail server has actively rejected the connection request. This immediate rejection, often distinct from a timeout, frequently stems from the recipient's server not having its mail service running or correctly configured to accept connections on port 25. Outbound firewalls on the sending server, or inbound firewalls at the recipient's end, also commonly block these crucial connections. Additionally, the sender's IP reputation plays a role, as many receiving servers will outright refuse connections from blacklisted or low-reputation IP addresses. Successfully troubleshooting this error demands a systematic approach, involving checks of network connectivity, both sender and recipient firewall rules, the operational status of mail server processes, and the sender's IP health.
Key opinions
- Mail Service Configuration: The error often points to the remote mail server's service, such as Postfix or Exim, not actively listening on the standard port 25, or not being configured to listen on the correct network interface.
- Firewall Intervention: Connection refused frequently occurs due to firewalls- either blocking outbound port 25 from the sender's server via tools like iptables or cloud security groups, or blocking inbound connections at the recipient's server.
- Active Rejection, Not Timeout: The presence of a TCP RST packet distinguishes connection refused as an active rejection by a process or firewall, rather than a passive network timeout, providing a clearer diagnostic signal.
- Reputation-Based Blocks: A recipient's server may actively refuse connections from sending IP addresses that are blacklisted or possess a poor reputation due to historical spamming or suspicious activity.
Key considerations
- Test Connectivity with Telnet: Utilize telnet to directly test if the remote mail server is reachable and listening on port 25, providing immediate insight into basic network and port availability.
- Verify Mail Server Process: Confirm that the mail transfer agent, such as Postfix or Exim, is running on the recipient's server and properly configured to listen on all necessary interfaces and port 25. Tools like netstat -tunlp can help verify this.
- Inspect Firewall Rulesets: Thoroughly check firewall configurations on both the sending server, specifically outbound rules for port 25, and investigate potential inbound blocks on the recipient's side, including network-level restrictions from hosting providers.
- Check Recipient Validity and Server Health: Ensure the recipient email address is valid and that the recipient's mail server domain is active, not experiencing known service outages, or being overloaded.
- Monitor Sender IP Reputation: Regularly assess your sending IP's reputation and check against common blacklists, as a poor reputation can lead to connection refusals by stricter receiving servers.
Marketer view
Email marketer from Mailgun Blog suggests that 'connection refused' indicates the recipient server is not allowing the connection, often due to an invalid recipient address, server downtime, or the recipient's server IP being blacklisted. They recommend verifying the recipient's email address, checking for known service outages, and reviewing your sending IP's reputation.
14 Aug 2024 - Mailgun Blog
Marketer view
Email marketer from Server Fault advises that 'connection refused' on port 25 points to the remote mail server not listening on that port or a firewall blocking it. They recommend using telnet <remote-IP> 25 to test connectivity and verifying that the remote mail server process (e.g., Postfix, Exim) is actually running and configured to listen on the correct network interface and port.
4 Jan 2022 - Server Fault
What the experts say
2 expert opinions
When an email delivery attempt results in a 'connection refused' error, it signifies an explicit rejection from the recipient's mail server. Beyond technical causes like firewalls or server outages, these errors can indicate a recipient's system is actively blocking a sender's IP, often due to poor reputation or perceived abuse. While troubleshooting involves checking common technical culprits, severe and persistent cases frequently necessitate direct engagement with the recipient's IT team, where senders must be prepared to justify their mailing practices, as these blocks can be notably challenging to reverse.
Key opinions
- Active Server Rejection: A 'connection refused' error signifies that the receiving server is actively and explicitly declining connections from the sending server, rather than a passive timeout or other error.
- Common Root Causes: This rejection can be due to various reasons, including the sender's IP address being blacklisted or rate-limited, a firewall blocking the connection on either side, the receiving mail server being down or overloaded, or incorrect DNS records pointing to an invalid server.
- Indicates Severe Blocking: Severe 'connection refused' blocks often suggest that the sender has significantly annoyed the recipient, typically after the recipient's system employed softer blocking methods that were overlooked or ignored.
- Difficult to Resolve: Such persistent and severe connection refusals are frequently difficult, if not impossible, to get lifted, emphasizing the need for proactive sender reputation management.
Key considerations
- Assess Scope: When encountering a 'connection refused' error, first determine the scope of the problem. Identify how many email addresses are affected, if the issue is specific to certain IP addresses, and how long the problem has persisted to help narrow down the potential causes.
- Contact Recipient IT: For severe or persistent 'connection refused' errors, directly contacting the recipient's IT team or email administrator is often the most direct and, at times, the only viable path to resolution. They can investigate blocks on their end.
- Justify Your Mail: Be prepared to explain why your email should be allowed through. Such severe blocks often suggest the sender has 'really annoyed' the recipient, frequently after softer blocking methods were ignored, so a clear justification is essential.
- Verify IP Reputation: Check your sending IP's reputation and consult common blacklists. A poor reputation or blacklisting can lead to recipient servers outright refusing connections from your IP address.
- Validate MX Records: Ensure the recipient's MX (Mail Exchange) DNS records are accurate and correctly point to a legitimate, active mail server for their domain.
- Isolate with New IP: If feasible, attempt to send from a different IP address. This can help isolate whether the 'connection refused' error is tied to the reputation of your specific sending IP or is indicative of a broader issue with the recipient's server or network.
Expert view
Expert from Email Geeks explains that when encountering a 'connection refused' error, you should first determine the scope of the problem, such as how many email addresses are affected, if it's specific IPs, and how long the issue has persisted. She advises that contacting the recipient's IT team is typically the only viable option. However, she notes that such severe blocks usually indicate the sender has 'really annoyed' the recipient, often after softer blocking methods were ignored. Therefore, be prepared to justify why your mail should be allowed through, and understand that these types of blocks are often difficult, if not impossible, to get lifted.
28 Apr 2023 - Email Geeks
Expert view
Expert from Word to the Wise explains that a 'connection refused' error means the receiving server is not accepting connections from the sending server. This can happen due to various reasons, including the sender's IP address being blacklisted or rate-limited, a firewall blocking the connection, the receiving mail server being down or overloaded, or incorrect DNS records pointing to a non-existent or wrong server. To troubleshoot, senders should check their IP reputation and blacklists, verify the recipient's MX records for accuracy, and, if necessary, contact the recipient's email administrator or attempt sending from a different IP to isolate the issue.
28 Feb 2022 - Word to the Wise
What the documentation says
5 technical articles
A 'connection refused' email delivery error indicates an explicit decision by the recipient's mail server or network to block the incoming connection. This direct rejection often stems from the remote mail server's service not running or being improperly configured to accept connections, network path issues preventing the connection, or firewalls on either the sending or receiving end actively blocking the attempt. Effective troubleshooting requires systematic checks of server status, network configurations, and security policies.
Key findings
- MTA Service Inactivity: A prevalent cause is the recipient's Mail Transfer Agent (MTA) software, such as Postfix or Exim, being inactive, offline, or incorrectly configured not to listen for incoming connections on standard ports like 25.
- Bilateral Firewall Blocks: Firewalls on either the sending server (outbound) or the recipient's server (inbound), including tools like UFW, iptables, or CSF, frequently prevent the necessary SMTP connection from establishing.
- Network Path & DNS Anomalies: Underlying network connectivity issues, incorrect DNS configurations, especially MX records pointing to an unavailable or wrong server, can lead to connection refusal by making the destination unreachable.
- Sender Reputation as a Factor: The recipient's server may actively refuse connections from sending IP addresses that are blacklisted or have a poor reputation, signaling a deliberate block based on perceived threat.
Key considerations
- Verify Mail Server Operation: Confirm that the recipient's mail server service is actively running and properly configured to accept connections on port 25, as its inactivity is a common source of the error.
- Check Network & Port Accessibility: Utilize diagnostic tools like telnet or traceroute to test direct connectivity to the remote server on port 25, helping to identify network path or port-blocking issues.
- Review Firewall Rules: Thoroughly examine firewall configurations on both your sending server and the recipient's network perimeter to ensure that inbound and outbound rules permit SMTP traffic on port 25.
- Validate DNS Records: Ensure that the recipient domain's MX (Mail Exchange) records are correctly configured and point to an active, legitimate mail server.
- Consult Mail Server Logs: Examine your mail server's logs (e.g., Exim, Postfix) for specific error codes or messages that provide more detailed insights into why the connection was refused.
- Assess Sender IP Reputation: Regularly monitor your sending IP's reputation and check against common blacklists, as a compromised reputation can lead to outright connection refusals from recipient servers.
Technical article
Documentation from Postfix.org explains that 'connection refused' often indicates that the remote machine's Postfix server (or other MTA) is not running, or a firewall is blocking the connection, preventing the connection request from reaching the service. It advises checking network connectivity, firewall rules on both ends, and ensuring the mail server service is active on the remote host.
7 Sep 2023 - Postfix.org
Technical article
Documentation from Microsoft Learn details that a 'connection refused' error can stem from network connectivity issues, incorrect DNS records, or firewall/antivirus software blocking the connection on either the sender's or recipient's side. It suggests verifying network paths, checking server configurations for correct ports, and temporarily disabling firewalls for testing.
5 Apr 2023 - Microsoft Learn
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