Linux上Sendmail经常由于一些配置问题，导致邮件发送失败，下面整理、收集了一些邮件发送失败、异常的案例。
案例1：在新服务器上测试sendmail发送邮件时，发现邮件发送不成功，检查/var/log/maillog日志文件发现如下错误（Notice：hostname、邮箱地址等信息使用xxx代替）
tail /var/log/maillog
May 15 09:43:38 xxxxx sendmail[9182]: s4F1hcQe009182: from=root, size=58269, class=0, nrcpts=1, msgid=<201405150143.s4F1hcQe009182@xxxx>, relay=root@localhost
May 15 09:43:38 xxxxx sendmail[9182]: s4F1hcQe009182: to=konglb@xxxx, delay=00:00:00, mailer=esmtp, pri=88269, dsn=4.4.3, stat=queued
May 15 09:43:58 xxxxx sendmail[9186]: NOQUEUE: SYSERR(oracle): can not chdir(/var/spool/mqueue/): Permission denied
出现这个问题，是因为oracle账号没有权限访问/var/spool/mqueue，需要授予相关权限，如下所示：
chmod 755 /var/spool/mqueue
案例2：查看var/log/maillog日志文件发现如下错误
May 15 10:21:41 xxxxx sendmail[9850]: s4F2LfrT009850: to=<konglb@xxxx>, delay=00:00:00, mailer=esmtp, pri=37549, dsn=4.4.3, stat=queued
出现这个问题是因为dns server不正确设置导致，需要修改/etc/resolv.conf 下的nameserver值。
案例3：检查/var/log/maillog发现如下信息，邮件状态为Sent（stat=Sent），但是依然没有收到邮件。
[root@DB-Server mail]# tail /var/log/maillog
Dec 18 11:15:01 nbolnx01 sendmail[25312]: sBI3F18J025312: from=<oracle@xxxx.xxxx>, size=607, class=0, nrcpts=1, msgid=<201412180315.sBI3F12b025302@nbolnx01.xxxx.xxxx>, proto=ESMTP, daemon=MTA, relay=localhost.localdomain [127.0.0.1]
Dec 18 11:15:01 nbolnx01 sendmail[25302]: sBI3F12b025302: to=oracle, ctladdr=oracle (502/502), delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=30290, relay=[127.0.0.1] [127.0.0.1], dsn=2.0.0, stat=Sent (sBI3F18J025312 Message accepted for delivery)
Dec 18 11:15:01 nbolnx01 sendmail[25327]: sBI3F1mw025327: from=<oracle@xxxx.xxxx>, size=599, class=0, nrcpts=1, msgid=<201412180315.sBI3F1Ba025321@xxxx.xxxx>, proto=ESMTP, daemon=MTA, relay=localhost.localdomain [127.0.0.1]
Dec 18 11:15:01 nbolnx01 sendmail[25321]: sBI3F1Ba025321: to=oracle, ctladdr=oracle (502/502), delay=00:00:00, xdelay=00:00:00, mailer=relay, pri=30282, relay=[127.0.0.1] [127.0.0.1], dsn=2.0.0, stat=Sent (sBI3F1mw025327 Message accepted for delivery)
检查mailq，如下所示，发现大量邮件处于等待发送的队列
[root@DB-Server mail]# mailq
.................................................................
.................................................................
.................................................................
sBD2j1tq000440 214 Sat Dec 13 10:45 <oracle@xxxx.xxxx>
(host map: lookup (xxxx.com): deferred)
konglb@xxxx.com
sBD301x9000637 214 Sat Dec 13 11:00 <oracle@xxxx.xxxx>
(host map: lookup (xxxx.com): deferred)
konglb@xxxx.xxxx
Total requests: 120
[root@nbolnx01 mail]#
出現了"host map: lookup (domain): deferred"错误，意味着邮件的发送被延迟了。需要通过修改配置文件/etc/mail/sendmail.cf。找到 #O ResolverOptions=+AAONLY 这一行信息后将注释取消掉，然后重启sendmail服务即可解决问题。
[root@DB-Server ~]# service sendmail stop
Shutting down sm-client: [ OK ]
Shutting down sendmail: [ OK ]
[root@DB-Server ~]# service sendmail start
Starting sendmail: [ OK ]
Starting sm-client: [ OK ]
参考资料：
http://www.51osos.com/a/Linux_CentOS_RedHat/Linuxjichu/2010/1001/107.html
http://www.cnblogs.com/AloneSword/archive/2013/07/20/3203050.html
http://blog.chinaunix.net/uid-11888131-id-2804023.html