How to troubleshoot 'connection refused' email delivery errors?
Matthew Whittaker
Co-founder & CTO, Suped
Published 25 Jul 2025
Updated 19 Aug 2025
6 min read
Encountering a 'connection refused' error during email delivery can be incredibly frustrating. It means that the recipient's mail server actively rejected your attempt to connect. Unlike a 'connection timed out' error, which suggests network issues or server unresponsiveness, 'connection refused' indicates a deliberate denial.
This denial can stem from various reasons, from misconfigurations on your sending server to stringent security measures or reputation issues on the receiving end. When you see this error, your email simply isn't getting through, leading to missed communications and potential business impact.
Troubleshooting this issue requires a systematic approach, starting with your own setup and gradually moving to the recipient's server configurations. Understanding the root cause is essential for implementing a lasting solution and ensuring your emails reach their intended recipients.
Decoding the 'connection refused' error
When your email delivery fails with a 'connection refused' message, it's the server equivalent of a slammed door. This specific error signifies that the server you're trying to connect to explicitly rejected your connection request. It's not a matter of the server being busy or unreachable, but rather an active refusal to communicate.
This type of error often occurs if your SMTP client (or plugin) cannot establish a connection on the specified port. It can point to issues on both the sending and receiving ends, but more frequently, it indicates a block or configuration problem on the recipient's side designed to prevent unwanted connections.
Differentiating 'connection refused' from other errors, like a connection timeout, is crucial for effective troubleshooting. A timeout implies the server didn't respond within a set period, possibly due to network congestion or the server being down. Refusal, however, is an explicit rejection, signaling a higher barrier to entry for your emails.
Common culprits behind the refusal
Several factors can lead to a 'connection refused' error. One of the most common causes is a firewall, either on your sending server or the recipient's. Firewalls are designed to block suspicious or unauthorized traffic, and if your IP address or port is deemed problematic, the connection will be refused.
Incorrect SMTP settings are another frequent culprit. If the hostname or port number for the outgoing mail server is misspelled or configured incorrectly, your client will try to connect to the wrong place or port, resulting in a refusal. This often happens after migrations or updates to your email infrastructure.
Furthermore, if the recipient's mail server isn't running or listening on the expected port, any connection attempt will be refused. This could be due to maintenance, a crash, or misconfiguration on their end. Lastly, active IP blocklisting (or blacklisting) of your sending IP by the recipient's server or a Real-time Blackhole List (RBL) can also trigger a 'connection refused' error as a direct denial mechanism.
Common causes
Firewall blocks: Your network firewall or the recipient's server firewall (which might include a blocklist) preventing connections on specific ports.
Incorrect SMTP configuration: Wrong SMTP host, port, or security settings in your email client or application.
Recipient server issues: The mail server is down, overloaded, or not listening on the expected port.
IP reputation/blocklisting: Your sending IP address is on a public or private blacklist or blocklist, causing the recipient to refuse connections from it.
Step-by-step troubleshooting guide
My first step in troubleshooting a 'connection refused' error is always to verify the basics. Start by checking your network connectivity to the target mail server. A simple ping or traceroute can confirm if the recipient's domain is reachable at all. If not, the issue might be broader than just email.
Next, confirm your SMTP server settings, including the host, port, and authentication credentials. Even a minor typo can cause a connection refusal. The most common SMTP ports are 25 (standard, often blocked), 587 (submission, with TLS encryption), and 465 (SMTPS, encrypted). Ensure you're using the correct one for your provider.
A useful diagnostic tool is telnet, which allows you to manually attempt a connection to the mail server on a specific port. If telnet also receives a 'connection refused' error, it strongly suggests a firewall or server-side issue. Here’s an example:
Test SMTP connection with TelnetBASH
telnet mail.example.com 25
If these initial checks don't resolve the issue, consult your email server logs. They often provide more detailed error messages or codes that can pinpoint the exact reason for the refusal. For instance, an Microsoft Exchange Online non-delivery report (NDR) might provide specific diagnostics.
Advanced diagnostics and prevention
If basic troubleshooting doesn't yield results, you'll need to delve deeper into advanced diagnostics. This often involves checking your sender reputation and whether your IP or domain is listed on any public blacklists (or blocklists). Many recipient servers use these lists to refuse connections from known spam sources. Even if you're not on a public list, some organizations maintain private blocklists based on their internal filtering policies.
Reviewing your email authentication records—SPF, DKIM, and DMARC—is also critical. Misconfigured or missing records can signal suspicious activity to receiving servers, leading them to refuse connections. For instance, a failure in DMARC verification can result in outright rejection or 'connection refused' at the SMTP handshake.
For prevention, consistent monitoring of your email deliverability and sender reputation is key. Regularly check your Google Postmaster Tools data and any available DMARC reports. Maintaining a clean sending list, avoiding spam traps, and adhering to email best practices significantly reduce the likelihood of future 'connection refused' errors. If you face persistent issues with a specific domain, contacting their IT department with detailed logs can sometimes lead to resolution, though this is often a last resort.
Views from the trenches
Best practices
Maintain a healthy email sending reputation by ensuring consistent sender authentication.
Regularly clean your mailing lists to remove inactive or problematic email addresses.
Monitor email bounce messages and logs diligently for early warning signs of delivery issues.
Common pitfalls
Ignoring soft bounce messages or temporary failures which can escalate to hard blocks.
Failing to check if your sending IP is on public or private blocklists regularly.
Assuming connection refused is always a network issue, overlooking sender reputation problems.
Expert tips
Before contacting a recipient's IT team, gather all relevant logs and an explanation of your email sending practices.
Understand that an active 'connection refused' typically means the recipient server has a strong reason for the block.
Consider a new dedicated IP address if your current one has a severely damaged reputation that cannot be salvaged.
Expert view
Expert from Email Geeks says that determining the volume of affected email addresses is important before deciding if further troubleshooting is worthwhile.
November 12, 2019 - Email Geeks
Expert view
Expert from Email Geeks says that understanding whether the connection refusal affects all your IPs or just some can help pinpoint the issue.
November 12, 2019 - Email Geeks
Summary of resolution strategies
While a 'connection refused' error can be a stubborn deliverability challenge, it's often a clear signal that specific actions are needed. By methodically checking your SMTP configuration, confirming network accessibility, and reviewing server logs, you can usually identify the immediate cause.
However, the most persistent 'connection refused' issues usually point to underlying sender reputation problems or being placed on a blacklist. Proactive measures, like rigorous list hygiene and consistent authentication, are your best defense. Addressing these issues not only resolves current blocks but also fortifies your email deliverability for the long term, ensuring your messages reach the inbox reliably.
Microsoft Exchange Online non-delivery report (NDR) might provide specific diagnostics.
Google Postmaster Tools data and any available DMARC reports. Maintaining a clean sending list, avoiding spam traps, and adhering to email best practices significantly reduce the likelihood of future 'connection refused' errors. If you face persistent issues with a specific domain, contacting their IT department with detailed logs can sometimes lead to resolution, though this is often a last resort.
