I have been testing this plugin and have set up and deleted many newsletters. The ID is therefore growing and growing, even though I have not used any of the newsletters as yet.
Which file and or folder do I access in order to reset the Newsletter IDs back to zero?
You should run the truncate dommand on the wp_newsletter table directly in the database. There is no other way. Or remove completely the plugin using the “remove all” button in main setting panel (but you’ll loose every configuration as well).