邮件发送后被退回。原因：
1.对方地址输错
2.对方退回邮件
3.DNS故障
4.其他故障
退回邮件中有“size exceed; data exceed”语句为对方邮箱已满或附件过大拒收，像“HOST NOT FIND，COMMUNICATION FAILURE”需查找具体原因。以本机(windows2003)为例,在DOS命令符下输入：
D:\Documents and Settings\Administrator>nslookup
Default Server:  dnsserver.eliteon-lis.com.cn
Address:  192.168.20.5
> kingmax.com.cn
Server:  dnsserver.eliteon-lis.com.cn
Address:  192.168.20.5
Non-authoritative answer:
Name:    kingmax.com.cn
Addresses:  192.168.1.2, 59.41.101.54, 59.41.101.46
> server 168.95.1.1 （换个外网DNS比较一下，这个是台北DNS)
Default Server:  dns.hinet.net
Address:  168.95.1.1
> kingmax.com.cn
Server:  dns.hinet.net
Address:  168.95.1.1
Name:    kingmax.com.cn
Addresses:  59.41.101.54, 59.41.101.46, 192.168.1.2
 
> set type=mx (通常进入NSLOOKUP，默认操作类型为A纪录.但是我们要解决的是邮件问题，当然需要查看的是邮件MX纪录,也可用set q=mx ，切换到A记录用set q=A)
> kingmax.com.cn
Server:  dns.hinet.net
Address:  168.95.1.1
DNS request timed out.   (第一次解析有延时，所以TIME OUT，多试几次）
    timeout was 2 seconds.
DNS request timed out.
    timeout was 2 seconds.
*** Request to dns.hinet.net timed-out
> kingmax.com.cn
Server:  dns.hinet.net
Address:  168.95.1.1
kingmax.com.cn  MX preference = 10, mail exchanger = mail01.kingmax
mail01.kingmax.com.cn   internet address = 59.41.101.54
以上可以看出邮件纪录会发送给 mail01.kingmax.com.cn这台服务器。
telnet mail01.kingmax.com.cn 25 （登录邮件服务器发送端口25）
220 wpemail.kingmax.com.cn Microsoft ESMTP MAIL Service, Version: 6.0.3790
ready at  Mon, 21 Jul 2008 17:03:19 +0800  （220表示成功）
helo
250 w2k305.eliteon.com Hello [59.41.101.35] （250表示正常）
mail from:rubar_luo@kingmax.com.cn
250 2.1.0 [email]rubar_luo@kingmax.com.cn[/email]....Sender OK
rcpt to:rubar_luo@eliteon.com
250 2.1.5 [email]rubar_luo@eliteon.com[/email]
data
354 Start mail input; end with <CRLF>.<CRLF>  (邮件内容结束时需要以" . "（半角句号）+回车 结束)
 subject:This is a test mail  (subject: 表示后面是邮件的标题,两次回车)
 
 
as the title.

 .                 (回车后输入 . 号)


.
250 2.6.0 < [email]W2K305R0Hy9Ngv2LHD300006ecc@w2k305.eliteon.com[/email]> Queued mail for deliv
ery
quit            (退出TELNET)
221 2.0.0 w2k305.eliteon.com Service closing transmission channel
继续看接收邮件端口
telnet mail01.kingmax.com.cn 110
+OK Microsoft Exchange Server 2003 POP3 伺服器版本 6.5.7226.0 (wpemail.kin
om.cn) 可以使用。
user rubar_luo
+OK
pass 邮箱密码
+OK User successfully logged on.
list
+OK 1 1637
1 1637    （左边为邮件编号，右边为邮件字节大小）
.
retr 1（显示邮件内容）
+OK
Received: by wpemail.kingmax.com.cn
        id < [email]01C8EB9A.838E3E8C@wpemail.kingmax.com.cn[/email]>; Tue, 22 Jul 2
 +0800
MIME-Version: 1.0
Content-Type: multipart/report;
        report-type=delivery-status;
        boundary="----_=_NextPart_001_01C8EB9A.838E3E8C"
X-DSNContext: 335a7efd - 4523 - 00000001 - 80040546
X-MimeOLE: Produced By Microsoft Exchange V6.5.7226.0
Content-class: urn:content-classes:dsn
Subject: =?gb2312?B?zrTLzbTvOoL332ag7pFCzajWqiAgKMqnlKEp?=
Date: Tue, 22 Jul 2008 09:30:24 +0800
Message-ID: < [email]LwXP8DEAC0000316a@wpemail.kingmax.com.cn[/email]>
X-MS-Has-Attach: yes
X-MS-TNEF-Correlator:
thread-index: AcjrmoOORzLR7wjaTwmSlt9ay+RTNAAAAAAK
From: =?gb2312?B?z7XNs7ncwO3UsQ==?= < [email]postmaster@kingmax.com.cn[/email]>
To: "rubar luo" < [email]rubar_luo@kingmaxchina.com[/email]>