Troubleshooting Sendmail Error 110: Connection Timed Out

Encountering errors when setting up Sendmail for the first time can be frustrating, especially when emails fail to arrive, not even in the spam folder. One common issue is the “Transport/null] Error 110: Connection Timed Out”. This error message, often found in mail logs, indicates a problem with Sendmail’s ability to connect to the destination mail server. Let’s explore what this error means and how to troubleshoot it.

Understanding “Connection Timed Out” in Sendmail

The “Connection timed out” error signifies that Sendmail attempted to establish a connection with a remote mail server but failed to do so within a specific timeframe. In the context of the provided logs, the error stat=Deferred: Connection timed out with alt4.gmail-smtp-in.l.google.com. clearly shows Sendmail’s inability to connect to Google’s mail server (gmail-smtp-in.l.google.com). This deferral means Sendmail will attempt to resend the email later, but the initial attempt failed due to a timeout.

Common Causes of Sendmail Error 110

Several factors can lead to this connection timeout issue. Here are some of the most frequent causes:

• Firewall Restrictions: A firewall on your server or network might be blocking outgoing connections on port 25 (SMTP), which is essential for sending emails.
• DNS Resolution Problems: Sendmail needs to resolve the recipient mail server’s domain name (e.g., alt4.gmail-smtp-in.l.google.com) to an IP address. DNS resolution failures can prevent connection establishment.
• Network Connectivity Issues: General network problems, such as internet outages or routing issues, can hinder Sendmail’s ability to reach the destination server.
• Recipient Server Issues: Although less likely in this scenario (as Gmail servers are generally reliable), the recipient’s mail server could be temporarily down or experiencing issues.
• Sendmail Configuration Errors: While less directly related to “connection timed out,” misconfigurations in Sendmail might indirectly lead to connection problems.

Troubleshooting Steps to Resolve Error 110

To diagnose and fix the “connection timed out” error, follow these troubleshooting steps:

1. Examine Mail Logs: As you’ve already done, the mail.log is crucial. Look for entries related to the deferred message and specifically the “Connection timed out” error. The log snippet provided in the original post clearly points to this issue:

   Feb 17 17:00:09 mysite sm-mta[689]: w1HGiDDr003604: to=<[email protected]>, ctladdr=<[email protected]> (33/33), delay=00:15:56, xdelay=00:10:01, mailer=esmtp, pri=120652, relay=alt4.gmail-smtp-in.l.google.com. [74.125.28.26], dsn=4.0.0, stat=Deferred: Connection timed out with alt4.gmail-smtp-in.l.google.com.

2. Verify Port 25 is Open: Use netstat to confirm that port 25 is indeed open and that Sendmail is listening on it. The output from netstat -tuplen | grep 25 as shown:

   tcp        0      0 127.0.0.1:25            0.0.0.0:*               LISTEN      687/sendmail: MTA:

   indicates that Sendmail is listening on port 25, but only on the loopback interface (127.0.0.1). For external mail delivery, ensure Sendmail is configured to listen on your server’s public IP or 0.0.0.0 for all interfaces. Check your sendmail.mc configuration file, specifically the DAEMON_OPTIONS settings.

3. Test DNS Resolution: Use ping or dig to check if your server can resolve the destination mail server’s domain name. For Gmail, try:

   ping alt4.gmail-smtp-in.l.google.com

   or

   dig alt4.gmail-smtp-in.l.google.com MX

   Successful DNS resolution is necessary for Sendmail to find the IP address of the mail server.

4. Check Firewall Settings: Investigate your server’s firewall (e.g., iptables, firewalld, or cloud provider firewalls). Ensure that outgoing traffic on port 25 (TCP) is allowed. This is a very common cause of connection timeouts.

5. Review Sendmail Configuration: Examine your Sendmail configuration files, particularly sendmail.mc and potentially hosts and hostname. While the provided sendmail.mc seems relatively standard, ensure no configurations are explicitly blocking outgoing connections or causing DNS resolution issues. Double check that your hostname is correctly set and resolvable.

Conclusion

The “transport/null] error 110: connection timed out” in Sendmail points to a failure in establishing a network connection with the destination mail server. By systematically checking firewall rules, DNS resolution, network connectivity, and Sendmail’s listening configuration, you can effectively diagnose and resolve this issue, ensuring your emails are successfully delivered. Start with the most likely culprits like firewalls and DNS, and methodically work through the troubleshooting steps to pinpoint the root cause.