qmail is the great SMTP server, written by Dan Bernstein.
Reference site for qmail is http://www.qmail.org.
Reference site for Dan Berstein is http://cr.yp.to/djb.html
vpopmail has been originally developed as opensource package by Ken Jones of Inter7 http://www.inter7.com.
Now is an opensource project hosted by sourceforge (http://vpopmail.sourceforge.net) and lead by Ken Jones, Tom Collins and Michael Bowe.
Bill Shupp toaster is an excellent integration of important qmail patches.
Bill Shupp is the developer and mantainer of the toaster, and http://www.shupp.org is the reference site for the toaster.
The mail toaster is a collection of open-source software which provides a full-featured mail server, tailored around qmail, running on FreeBSD.
Matt Simerson is the primary author and maintainer of the toaster. There is an active and friendly community of toaster owners which supports the toaster on a mailing list and web forum on http://www.thenetworkpeople.biz/internet/mail/toaster/.
chkuser & chkusr copyrights
chkuser is developed and mantained by Antonio Nati (http://opensource.interazioni.it/).
Check in disclaimer & copyrights section for disclaimer and copyright informations.