I have followed the tutorial(exactly) http://wiki.centos.org/HowTos/postfix
but I can't telnet my server's 25 port, I have no idea about this issue, so why?
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you check at /var/log/ and any firewall rule and selinux disable and have look following link http://studyhat.blogspot.com/2009/10/postfix-server.html
Can you telnet to the mail server's port 25 using localhost (from that machine itself)?
If not, can you see the smtpd process running with the ps command?
If it's running and you're not seeing any errors in /var/log for the process, then the next thing to check is that you don't have firewall rules (iptables?) blocking access to the port, or if you can connect locally to 25 but not from another machine outside your network that you don't have a firewall that is blocking access. You need to have port 25 forwarded to the proper internal machine.
Nmap from an outside computer may help tell you what ports are open and visible from other machines. sockstat or socklist can also help on that machine. Check your local logs in /var/log to see if there are errors showing up in the firewall or SMTP processes.
Try these steps:
chkconfig --list postfix(it should be enabled in runlevel 3)
chkconfig postfix on
netstat -tlpen | grep 25to check if Postfix is listening on port 25
service postfix start, then check if error messages appear in /var/log/maillog
getenforcereturns "Enforcing", then it is
service setroubleshoot startthen watch its logfile /var/log/audit/audit.log for errors concerning Postfix