http://blog.csdn.net/buster2014/article/details/46377725 (CentOS install sendmail and settings)
http://blog.chinaunix.net/uid-20282925-id-86216.html (Sendmail installation notes with authentication)
http://dgd2010.blog.51cto.com/1539422/1421904/ (关于sendmail报错“did not issue MAIL/EXPN/VRFY/ETRN during connection to MTA)