linux – 无法连接到SMTP服务器

telnet mail.example.com 25
Trying 12.23.34.45...
telnet: Unable to connect to remote host: Connection timed out

来自我网络外部的另一台Debian PC：

telnet mail.example.com 25
Trying 12.23.34.45...
Connected to mail.example.com.
Escape character is '^]'.
220 mail.example.com ESMTP Exim 4.63 Mon,02 Apr 2012 22:14:09 +0200

ping也永远挂起,当我按下Ctrl C时它会说“100％丢包”.
最后,traceroute充满了星号：

traceroute mail.example.com
traceroute to mail.example.com (12.23.34.45),30 hops max,60 byte packets
1  * * *
2  * * *
3  * * *
...
27  * * *
28  * * *
29  * * *
30  * * *

当我对服务器端口25运行nmap时：

nmap mail.example.com -p25

Starting Nmap 5.21 ( http://nmap.org ) at 2012-04-02 23:08 EEST
Nmap scan report for mail.example.com (12.23.34.45)
Host is up (0.090s latency).
PORT   STATE    SERVICE
25/tcp filtered smtp

Nmap done: 1 IP address (1 host up) scanned in 0.88 seconds

最后我的iptables：

iptables -L
Chain INPUT (policy ACCEPT)
target     prot opt source               destination         

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination         

Chain OUTPUT (policy ACCEPT)
target     prot opt source               destination

有任何想法吗？
非常感谢您的帮助.