我们常常会遇到各种退信，标题为“永久投递失败”或是“暂时投递失败”，我们先看一封系统账户发送回来的短暂投递失败的退信。如下日志显示。

  From: "MDaemon at a.com"
 To:
 Sent: Thursday, December 30, 2010 10:36 AM
 Subject: 短暂投递失败

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MDaemon 投递状态通知 - http://www.altn.com/dsn
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所附邮件有“暂时性”非致命投递错误。

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这只是一则警告邮件 - 您不必重新发送邮件
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MDaemon 配置为按设定的时间间隔自动
重试投递。后续投递该邮件的尝试正待处理。

--- Session Transcript ---
Thu 2010-12-30 10:35:22: Parsing message
Thu 2010-12-30 10:35:22: *  From: i*@a.com
Thu 2010-12-30 10:35:22: *  To: s*@b.com
Thu 2010-12-30 10:35:22: *  Subject:=?gb2312?B?UmU6IEVUMTAtMDctMDA4ICAgIE1BU1Qgv827p8ihz/uzydLCtaW68w==?==?gb2312?B?tbzWwiAgydzQy9mkwurM2Lmry761xL/itObD5sHPIA==?=
Thu 2010-12-30 10:35:22: *  Size (bytes): 28763
Thu 2010-12-30 10:35:22: *  Message-ID:<25B6FE5232BE423780A8636A6FB0315B@I*>
Thu 2010-12-30 10:35:22: *  Route slip host: b.com Thu 2010-12-30 10:35:22: *  Route slip port: 25
Thu 2010-12-30 10:35:22: Attempting SMTP connection to [b.com] Thu 2010-12-30 10:35:23: Resolving MX records for [b.com] (DNS Server:202.96.209.5)...
Thu 2010-12-30 10:35:33: *  DNS: 10 second wait for DNS response exceeded (attempt 1 of 3) Thu 2010-12-30 10:35:33: Resolving MX records for [b.com] (DNS Server:202.96.209.133)...
Thu 2010-12-30 10:35:43: *  DNS: 10 second wait for DNS response exceeded (attempt 2 of 3) Thu 2010-12-30 10:35:43: Resolving MX records for [b.com] (DNS Server:202.96.209.5)...
Thu 2010-12-30 10:35:53: *  DNS: 10 second wait for DNS response exceeded (attempt 3 of 3) Thu 2010-12-30 10:35:53: Attempting SMTP connection to [b.com:25] Thu 2010-12-30 10:35:53: Resolving A record for [b.com] (DNS Server:202.96.209.5)...
Thu 2010-12-30 10:36:03: *  DNS: 10 second wait for DNS response exceeded (attempt 1 of 3) Thu 2010-12-30 10:36:03: Attempting SMTP connection to [b.com:25] Thu 2010-12-30 10:36:03: Resolving A record for [b.com] (DNS Server:202.96.209.133)...
Thu 2010-12-30 10:36:13: *  DNS: 10 second wait for DNS response exceeded (attempt 2 of 3) Thu 2010-12-30 10:36:13: Attempting SMTP connection to [b.com:25] Thu 2010-12-30 10:36:13: Resolving A record for [b.com] (DNS Server:202.96.209.5)...
Thu 2010-12-30 10:36:23: *  DNS: 10 second wait for DNS response exceeded (attempt 3 of 3) Thu 2010-12-30 10:36:23: 此邮件已经过了 61 分；在这个队列还剩 0 分
Thu 2010-12-30 10:36:23: 远程队列生命期已超过；邮件放到重试队列
--- End Transcript ---

    这封退信其实是告诉发件人 i*@a.com，之前发送给s*@b.com的邮件，在远程队列的时间已经超过了正常响应的时间，采取的操作是邮件放到重试队列里了，并且服务器隔一段时间之后会重新发送邮件。除了告诉我们这些，上面日志加粗深红色的部分说明在进行dns解析时出现问题。

    解决方法：首先ping dns服务器，如果正常返回，则可能是解析的问题，建议更换dns（设置---默认域服务器---dns服务器---取消勾选 尝试使用windowsTCP/IP设置中的DNS，下面填写其它dns服务器地址即可）；如果无法正确返回信息，则有可能是联系不上dns，联系不上dns建议检查内网到dns服务器的连通性。