How everything works:
When you start this daemon it does two main things:
1. it will start spawning the send-campaigns command from your Mailwizz APP, that is why you need to stop any cron that does this already
2. it will wait for emails to be written in the pickup directory so that it can start sending them using the servers you defined in config.json file
When a send-campaigns command is triggered by this daemon, it will choose a delivery server to send your email campaigns. While the campaign is running, the emails are actually written in the pickup directory instead of sending them directly. Because the daemon watches the pickup directory, when new emails are added, the daemon will pick them and will use the delivery servers you defined in the config.json file to do the actual delivery.
Doing like this we make sure Mailwizz doesn't have to wait for the actual delivery to happen, it's solely purpose is to write the emails in the pickup directory and then it's this daemon job to send them.