Here is what you need to get the WordPress Mailing Group plugin working on your WordPress website:
1. An email address for each mailing group you want to set up, for example: firstname.lastname@example.org
If you have access to an online control panel for your domain name & website, you can likely set one up there. This should NOT be a forwarding address, but a real POP or IMAP mailbox.
2. Optional access to the Cron Jobs, also called Scheduled Tasks, on your web server, so you can add a line to that list. The plugin runs with the in-built WP-cron, but it requires for a visitor to come to your site to make that WP-cron run its scripts. If you have a low traffic site, this may mean that messages to and from your mailing group and not received or sent often enough for your preferences. With an external Cron Job, you can call the WP-cron art regular intervals, eg. every two minutes, to make it run the Mailing Group more smoothly.