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OK so this is driving me crazy.

Running sendmail on CentOS. It's running, and listening on 25:

[root@mysqlslavebackup ~]# nmap -sS -O -sV 127.0.0.1

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2013-03-21 10:46 PDD<br>
Interesting ports on localhost.localdomain (127.0.0.1):<br>
Not shown: 1665 closed ports<br>
PORT      STATE SERVICE           VERSION<br>
22/tcp    open  ssh               OpenSSH 4.3 (protocol 2.0)<br>
25/tcp    open  smtp              Sendmail 8.13.8/8.13.8<br>
53/tcp    open  domain<br>
111/tcp   open  rpc<br>
631/tcp   open  ipp               CUPS 1.2<br>
739/tcp   open  rpc<br>
953/tcp   open  rndc?<br>
3306/tcp  open  mysql             MySQL 5.5.11-enterprise-commercial-advanced-log<br>
5901/tcp  open  vnc               VNC (protocol 3.8)<br>
5902/tcp  open  vnc               VNC (protocol 3.8)<br>
5903/tcp  open  vnc               VNC (protocol 3.8)<br>
6001/tcp  open  X11                (access denied)<br>
6002/tcp  open  X11                (access denied)<br>
6003/tcp  open  X11                (access denied)<br>

I can telnet to 25 from the localhost, but from any other machine it appears the port is closed. iptables are off, and even if they were in I have an entry to open the port.

Can't for the life of me figure out what the heck it would be other than the fact this used to be a Zimbra mail server, but it has been uninstalled and there's no record of it in port bindings.

Any input would be awesome! Thanks!

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marked as duplicate by Kenny Rasschaert, dunxd, mdpc, Ward, Jenny D Mar 29 '13 at 9:12

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

2  
It could be blocked at the network firewall, or as answer posted by Michael suggests, Sendmail may not be listening on the public network interface. – Stefan Lasiewski Mar 21 '13 at 18:28
1  
Please post the output of lsof -i -P | grep sendmail. By default for CentOS sendmail only listens on localhost. – Mark Wagner Mar 21 '13 at 18:58
    
@MarkWagner netstat -ant| grep :25 gives tcp 0 0 127.0.0.1:25 0.0.0.0:* LISTEN – Christopher Glenn Schlägel Mar 21 '13 at 21:16
1  
It is only listening on loopback. Andrzej's answer is correct. – Mark Wagner Mar 21 '13 at 22:14

Why can't I receive external mail/incoming SMTP connections? [Sendmail and other MTA servers]

Are you sure your sendmail does not listen ONLY on loopback interface? [127.0.0.1:25]
It is default configuration on quite a few distributions.

You may use netstat -ant| grep :25 to chek it.

Sendail FAQ 4.22: Why can't I receive external mail?
In short: You may need to comment out DAEMON_OPTIONS line with 127.0.0.1 in your sendail.mc and recompile it into sendmail.cf.

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1  
To rebuild and recompile in CentOS, after you edit sendmail.mc you run /etc/mail/make followed by a service sendmail restart. – adamo Mar 21 '13 at 22:26

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