Marketing. Build a list from your customers, write from a template with merge fields for their name and details, and send.
Unsubscribes are handled for you: every email carries a link, and anybody who uses it is excluded from every future send without you having to remember. That is a legal requirement, not a courtesy.
Before your first campaign
Send it to yourself. Merge fields that look right in the editor look different with real data in them, and "Hi {first_name}" reaching four hundred people is the mistake everybody makes once.
This is not the same as invoice reminders
Reminders about money go through Settings → Reminders and are triggered by the invoice, not by a list. Do not chase debtors from a marketing campaign — the unsubscribe link would let them opt out of being chased.