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 features were added. At end of July 2022, the MDA (POP/IMAP server) was moved out of this implementation, to Dovecot. Since then, this mailserver is only an 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.