Enhanced Mail System Status Codes

用低成本的办法建设一个稳健的，高效的邮件系统应该是每个系统管理员的梦想。现实中太多企业的 IT 人员喜欢采用商业的解决方案了。 在调试反垃圾系统的过程中，再次温习 dsn(delivery status notifications) 的含义。

   Status codes thus consist of three numerical fields separated by ".".
   The first sub-field indicates whether the delivery attempt was
   successful (2= success, 4 = persistent temporary failure, 5 =
   permanent failure).  The second sub-field indicates the probable
   source of any delivery anomalies, and the third sub-field denotes a
   precise error condition, if known.

对系统管理员，需要检查 dsn 不等于 2.0.0 的邮件，来保证邮件系统没有被滥用。例如：

May 18 20:50:30 freelamp sendmail[6221]: n4ICmNJA006201: to=username@21cn.com, delay=00:01:59, xdelay=00:01:44, mailer=esmtp, pri=
181763, relay=mta.21cn.com. [59.36.102.53], dsn=4.0.0, stat=Deferred: Connection reset by mta.21cn.com.
May 18 21:24:16 freelamp sendmail[10978]: n4ICmNJA006201: to=user@21cn.com, delay=00:35:45, xdelay=00:00:03, mailer=esmtp, pri
=271763, relay=mta.21cn.com. [59.36.102.55], dsn=5.0.0, stat=Service unavailable

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