本周四临近下班时，接到一个客户的电话——其Exchange 2013 CU2系统终端用户无法使用 SMTP 方式发送邮件。当客户端（OLK或者Foxmail）发送邮件时，系统提示错误：
451 4.7.0 Temporary server error. Please try again later. PRX2
诊断时，通过telnet Server 25命令检测可以连接；通过客户端软件连接时，身份验证操作成功。同时，在故障期间伴随着无法使用POP3收取邮件。
毋庸置疑地，Exchange 2013服务器端服务均正常运行。
通过打开前端连接器的“详细日志”，检查后发现了问题的根源所在：DNS查询失败。参见下面红褐色部分文字。
2014-05-22T04:23:36.138Z,CAS-BJ-02\SMTP,08D14140A7CA0122,23,192.168.0.7:25,192.168.0.5:64375,>,451 4.7.0 Temporary server error. Please try again later. PRX2 ,
2014-05-22T04:23:36.170Z,CAS-BJ-02\SMTP,08D14140A7CA0122,24,192.168.0.7:25,192.168.0.5:64375,-,,Remote(SocketError)
2014-05-22T04:25:01.141Z,CAS-BJ-02\SMTP,08D14140A7CA0124,0,192.168.0.7:25,192.168.0.5:62597,+,,
2014-05-22T04:25:01.141Z,CAS-BJ-02\SMTP,08D14140A7CA0124,1,192.168.0.7:25,192.168.0.5:62597,*,SMTPSubmit SMTPAcceptAnySender SMTPAcceptAuthoritativeDomainSender AcceptRoutingHeaders,Set Session Permissions
2014-05-22T04:25:01.141Z,CAS-BJ-02\SMTP,08D14140A7CA0124,2,192.168.0.7:25,192.168.0.5:62597,>,"220 mail.alexchen.info Microsoft ESMTP MAIL Service ready at Thu, 22 May 2014 12:25:00 +0800",
2014-05-22T04:25:01.235Z,CAS-BJ-02\SMTP,08D14140A7CA0124,3,192.168.0.7:25,192.168.0.5:62597,<,EHLO ozkzitc.mobi,
2014-05-22T04:25:01.235Z,CAS-BJ-02\SMTP,08D14140A7CA0124,4,192.168.0.7:25,192.168.0.5:62597,>,250-mail.alexchen.info Hello [192.168.0.5],
2014-05-22T04:25:01.235Z,CAS-BJ-02\SMTP,08D14140A7CA0124,5,192.168.0.7:25,192.168.0.5:62597,>,250-SIZE 524288000,
2014-05-22T04:25:01.235Z,CAS-BJ-02\SMTP,08D14140A7CA0124,6,192.168.0.7:25,192.168.0.5:62597,>,250-PIPELINING,
2014-05-22T04:25:01.235Z,CAS-BJ-02\SMTP,08D14140A7CA0124,7,192.168.0.7:25,192.168.0.5:62597,>,250-DSN,
2014-05-22T04:25:01.235Z,CAS-BJ-02\SMTP,08D14140A7CA0124,8,192.168.0.7:25,192.168.0.5:62597,>,250-ENHANCEDSTATUSCODES,
2014-05-22T04:25:01.235Z,CAS-BJ-02\SMTP,08D14140A7CA0124,9,192.168.0.7:25,192.168.0.5:62597,>,250-STARTTLS,
2014-05-22T04:25:01.235Z,CAS-BJ-02\SMTP,08D14140A7CA0124,10,192.168.0.7:25,192.168.0.5:62597,>,250-AUTH LOGIN,
2014-05-22T04:25:01.235Z,CAS-BJ-02\SMTP,08D14140A7CA0124,11,192.168.0.7:25,192.168.0.5:62597,>,250-8BITMIME,
2014-05-22T04:25:01.235Z,CAS-BJ-02\SMTP,08D14140A7CA0124,12,192.168.0.7:25,192.168.0.5:62597,>,250-BINARYMIME,
2014-05-22T04:25:01.235Z,CAS-BJ-02\SMTP,08D14140A7CA0124,13,192.168.0.7:25,192.168.0.5:62597,>,250 CHUNKING,
2014-05-22T04:25:01.266Z,CAS-BJ-02\SMTP,08D14140A7CA0124,14,192.168.0.7:25,192.168.0.5:62597,<,MAIL FROM: <[email protected]>,
2014-05-22T04:25:01.266Z,CAS-BJ-02\SMTP,08D14140A7CA0124,15,192.168.0.7:25,192.168.0.5:62597,*,08D14140A7CA0124;2014-05-22T04:25:01.141Z;1,receiving message
2014-05-22T04:25:01.266Z,CAS-BJ-02\SMTP,08D14140A7CA0124,16,192.168.0.7:25,192.168.0.5:62597,>,250 2.1.0 Sender OK,
2014-05-22T04:25:01.313Z,CAS-BJ-02\SMTP,08D14140A7CA0124,17,192.168.0.7:25,192.168.0.5:62597,<,RCPT TO: <[email protected]>,
2014-05-22T04:25:01.313Z,CAS-BJ-02\SMTP,08D14140A7CA0124,18,192.168.0.7:25,192.168.0.5:62597,>,250 2.1.5 Recipient OK,
2014-05-22T04:25:01.360Z,CAS-BJ-02\SMTP,08D14140A7CA0124,19,192.168.0.7:25,192.168.0.5:62597,<,DATA,
2014-05-22T04:25:01.360Z,CAS-BJ-02\SMTP,08D14140A7CA0124,20,192.168.0.7:25,192.168.0.5:62597,>,354 Start mail input; end with <CRLF>.<CRLF>,
2014-05-22T04:25:01.407Z,CAS-BJ-02\SMTP,08D14140A7CA0124,21,192.168.0.7:25,192.168.0.5:62597,*,,Proxy destination(s) obtained from OnProxyInboundMessage event
2014-05-22T04:25:01.407Z,CAS-BJ-02\SMTP,08D14140A7CA0124,22,192.168.0.7:25,192.168.0.5:62597,*,,Message or connection acked with status Retry and response 451 4.4.0 DNS query failed. The error was: SMTPSEND.DNS.NonExistentDomain; nonexistent domain
进一步检查前端服务器的连接日志。显然地，使用了公网DNS服务器114.114.114.114来解析内部主机的名称。
2014-05-22T04:25:01.407Z,08D14140A7CA0125,SMTP,internalproxy,+,Undefined 00000000-0000-0000-0000-000000000000;QueueLength=<no priority counts>
2014-05-22T04:25:01.407Z,08D14140A7CA0125,SMTP,internalproxy,>,Non-existent domain reported by 114.114.114.114. [Domain:Result] = MBX-BJ-02.alexchen.info:InfoDomainNonexistent; MBX-BJ-03.alexchen.info:InfoDomainNonexistent; MBX-BJ-01.alexchen.info:InfoDomainNonexistent;
2014-05-22T04:25:01.407Z,08D14140A7CA0125,SMTP,internalproxy,-,Messages: 0 Bytes: 0 (The DNS query for ‘Undefined‘:‘internalproxy‘:‘00000000-0000-0000-0000-000000000000‘ failed with error : InfoDomainNonexistent)
在前端服务器的主DNS服务器指向了内部的DC集成的DNS服务器，辅助DNS服务器配置参数指向114.114.114.114。依据上述的日志输出信息判断，显然是首DNS服务器失效，服务器才会继而使用辅助DNS服务器执行查询。但是，使用NSLookup命令查询上述日志中出现的主机时，得到正确的查询结果。
至此，排错目标转向内部DNS服务器。
首先的操作是重启DNS服务器。服务重启之后，意外出现了，在正向查找区域中 alexchen.info 的DNS Zone不见了，值剩下 _msdcs zone。
其次，重建DNS zone，使用DNS记录的文本备份。
最后，确认DNS回复正常之后，重启前后端Exchange的传输服务。故障消失。
451 4.7.0 Temporary server error. Please try again later. PRX2 故障处理