The mail500.mailinglist.maillog file shows some sample debug output from
the mail500 program (locally added print statements) that shows something
of its processing logic. This can be valuable when trying to understand
why something doesn't work as expected, ESPECIALLY with regards to the
required mail500 source change required (see mail500 docs) to make it work
within your environment (i.e., static internal definitions).

In a production environment, those debug statements should be disabled.