qmail MTA which can send/receive the mail have two starting points viz qmail-smtp and qmail-inject.
Why do we have two such different interfaces for mail delivery???
Am going through the tutorial of qmail referenced from: http://www.nrg4u.com/qmail/the-big-qmail-picture-103-p1.gif
The two different starting points/interfaces that you are talking about have significance importances:
qmail-smtpd: is responsible for accepting the mails from external world. It listens on port 25 and accepts the mail following the smtp protocol.
whereas qmail-inject: is responsible for sending the mail maybe within same domain or to other domain. (basically we call it a MUA)
