现象：
    不能发邮件，报错无法连接邮件服务器
    telnet smtp.163.com 465端口，不通
    在同网段的其他服务器测试，可以。定位为此服务器的问题。
    检查iptables，策略为允许icmp和特定ip，其他拒绝，最后一条为：
    -A INPUT -j REJECT —reject-with icmp-host-prohibited
    打开tcpdump抓包
    tcpdump host smtp.163.com -i em3 # em3为外网网口
    看到icmp-host-prohibited字样
    增加一条iptables规则
    sudo iptables -I INPUT 7 -s 220.181.12.18/32 -j ACCEPT #为smtp.163.com的ip
    telnet 的问题解决，握手成功。

    midc@pha:~$ telnet 220.181.12.18 465 Trying 220.181.12.18… Connected to 220.181.12.18.

    Escape character is ‘^]’.

    Connection closed by foreign host.

    TCPdump：

    midc@pha:~$ sudo tcpdump host 220.181.12.18 -i em3

    tcpdump: verbose output suppressed, use -v or -vv for full protocol decode

    listening on em3, link-type EN10MB (Ethernet), capture size 262144 bytes

    11:16:24.796521 IP ubuntu-R920-2.midc.54598 > m12-18.163.com.urd: Flags [S], seq 3936977535, win 29200, options [mss 1460,sackOK,TS val 4002279548 ecr 0,nop,wscale 7], length 0

    11:16:24.833942 IP m12-18.163.com.urd > ubuntu-R920-2.midc.54598: Flags [S.], seq 2701804405, ack 3936977536, win 14480, options [mss 1460,sackOK,TS val 3382967984 ecr 4002279548,nop,wscale 7], length 0

    11:16:24.834049 IP ubuntu-R920-2.midc.54598 > m12-18.163.com.urd: Flags [.], ack 1, win 229, options [nop,nop,TS val 4002279586 ecr 3382967984], length 0

    11:16:39.740786 IP ubuntu-R920-2.midc.54620 > m12-18.163.com.urd: Flags [S], seq 1417531113, win 29200, options [mss 1460,sackOK,TS val 4002294492 ecr 0,nop,wscale 7], length 0

    11:16:39.777043 IP m12-18.163.com.urd > ubuntu-R920-2.midc.54620: Flags [S.], seq 83674110, ack 1417531114, win 14480, options [mss 1460,sackOK,TS val 3382982928 ecr 4002294492,nop,wscale 7], length 0