Sendmail: https://www.sendmail.org/
Latest version and documentation of Smail: https://www.smail.org/
Qmail source, documentation and supporting software: https://www.qmail.org/
Apache web server, related projects and documentation: https://www.apache.org/
Squid proxy server: https://www.nlanr.net/squid/
Sunsite: https://metalab.unc.edu/, ftp://metalab.unc.edu/
tproxy package: ftp://ftp.nlc.net.au/pub/linux/www/