postfix能正常启动，但无法发送邮件，
查看日志文件（/var/log/maillog):
May 28 08:19:46 linuxidc postfix/local[17523]: fatal: open lock file pid/unix.local: cannot open file: Permission denied
May 28 08:19:47 linuxidc postfix/master[17517]: warning: process /usr/libexec/postfix/cleanup pid 17521 exit status 1
May 28 08:19:47 linuxidc postfix/master[17517]: warning: /usr/libexec/postfix/cleanup: bad command startup -- throttling
May 28 08:19:47 linuxidc postfix/master[17517]: warning: process /usr/libexec/postfix/local pid 17523 exit status 1
May 28 08:19:47 linuxidc postfix/master[17517]: warning: /usr/libexec/postfix/local: bad command startup -- throttling
解决方法：
关闭selinux
[root@ ~]#vi /etc/selinux/config
修改SELINUX为：
SELINUX=disabled
重启postfix服务，即可正常发出。