When attempting to establish a telnet connection to port 25 on a CentOS server running PostFix, you're encountering the frustrating "Connection refused" error. This typically indicates one of several potential issues in your mail server configuration.
First, confirm PostFix is actually running:
sudo systemctl status postfix
If it's not active, start it with:
sudo systemctl start postfix sudo systemctl enable postfix
PostFix needs to be configured to listen on the correct interfaces. Examine your main.cf:
sudo nano /etc/postfix/main.cf
Look for these critical parameters:
inet_interfaces = all inet_protocols = all
Even with SELinux disabled, iptables might be blocking traffic. Verify and add rules if needed:
sudo iptables -L -n sudo iptables -A INPUT -p tcp --dport 25 -j ACCEPT sudo service iptables save
Before testing from another machine, verify locally:
telnet localhost 25
If this works but remote connections fail, your issue is network-related.
PostFix might not be binding to your external interface. Check:
netstat -tulnp | grep :25
You should see output similar to:
tcp 0 0 0.0.0.0:25 0.0.0.0:* LISTEN 1234/master
Use postconf to verify all settings:
postconf -n
Pay particular attention to these parameters:
mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128 192.168.1.0/24
If telnet still fails, try these diagnostic commands:
nc -zv your.server.ip 25 nmap -p 25 your.server.ip
- Service running (systemctl status postfix)
- Correct inet_interfaces setting
- Proper iptables rules
- Network interface binding
- Appropriate mynetworks configuration
First, confirm if Postfix is actually running and listening on port 25:
sudo systemctl status postfix
If inactive, start it with:
sudo systemctl start postfix sudo systemctl enable postfix
Use netstat or ss to verify Postfix is bound to port 25:
sudo netstat -tulnp | grep :25 # OR sudo ss -tulnp | grep :25
Expected output should show Postfix listening on 0.0.0.0:25 or your server's IP.
Even with SELinux disabled, iptables might block the connection. Add a rule:
sudo iptables -I INPUT -p tcp --dport 25 -j ACCEPT sudo service iptables save
For firewalld users:
sudo firewall-cmd --add-port=25/tcp --permanent sudo firewall-cmd --reload
Inspect /etc/postfix/main.cf for these critical settings:
inet_interfaces = all # or specific IP: # inet_interfaces = 192.168.1.100
After changes, reload Postfix:
sudo postfix reload
If telnet still fails, try these diagnostic commands:
# Check local connection telnet localhost 25 # Test from another machine using nc nc -zv your.server.ip 25 # Detailed port test nmap -p 25 your.server.ip
Check /etc/postfix/master.cf to ensure SMTP service is enabled:
smtp inet n - n - - smtpd
This line should NOT be commented out.
When all else fails, use tcpdump to analyze traffic:
sudo tcpdump -i eth0 port 25 -vv -n
Look for SYN packets coming in but no response, indicating a deeper network issue.