qpsmtpd 介绍
qpsmtpd 是一个灵活的SMTPd的守护进程，采用Perl语言编写。
Apart from the core SMTP features, all functionality is implemented in small
“extension plugins” using the easy-to-use object oriented plugin API. qpsmtpd
was originally written as a drop-in qmail-smtpd replacement, but now it also
includes an smtp forward and a postfix “backend”.