This module allows you to give Dolibarr the ability to “track” / “log” a number of things. It's often “forgotten”, but once it's activated you won't be able to do without it!
For example: on a customer's invoice file, you'll have a list of events relating to this invoice: whether you've already sent it, when, to whom, and so on.
Activate this module and go to its configuration.
Next, so that Dolibarr can periodically start checking the notifications you send, don't forget to activate the scheduled task “Send calendar reminders by email”.
You can do this here: Home > Administrative tools > Scheduled tasks > Send calendar reminders by email …
And if your host doesn't offer you access to cron or a “web” task scheduler, you can use the dolizen one…