一些有用的链接：
http://www.sendmail.org/%7Eca/email/auth.html
http://www.jonfullmer.com/smtpauth/
http://www.joreybump.com/code/howto/smtpauth.html
http://server.1und1.com/root_server/bs/suse72/howtos/1.html
http://info.ccone.at/INFO/Mail-Archives/redhat/Jan-2002/msg02936.html
https://www.youcl.com/info/6966
