http://man.qmail.jp/jman5/qmail-control.html
http://www.kobitosan.net/postfix/trans-2.1/jhtml/postconf.5.html
http://www.kobitosan.net/postfix/trans-2.2/jhtml/postconf.5.html
http://www.kobitosan.net/postfix/trans-2.3/jhtml/postconf.5.html