Confusing to have sample ponymail.yaml config separate from setup.py

Any additions to ponymail config items currently need to be added to both setup.py and ponymail.yaml.example.

It's easy for these files to get out of step.

It would be better to have a single source of information, perhaps a template file.

Compare setup.py with ponymail.yaml.example to identify the duplicated configuration items and how each file is consumed. Determine a single-source approach that preserves the example configuration and setup behavior; done means new config items no longer need to be maintained in both files and the two representations cannot drift.

Written by the indexing model from the issue text.