It was originally planned that this implementation will eventually be replaced by a different one, but that never happened (fully; see below). That is one of the reasons why the code quality of this implementation slowly declined over time when more and more quick fixes were added, so look at your own risk. One of my examples of a big ball of mud (not the worst one though). At end of July 2022, the MDA (POP/IMAP server) was moved out of this implementation, to Dovecot. Since then, this mailserver is only a MTA. The userauth.lua script is the authentication script for dovecot. Not all files in this directory are actually deployed in the same location/server.