Homepage: https://www.yireo.com/software/joomla-extensions/emailscheduler
The EmailScheduler component offers an API for emails to be sent with a delay. The component allows for third party extensions to store these emails in the EmailScheduler table, together with a specified delay. Next, the EmailScheduler component (via a cronjob) picks up these scheduled mails and sends them when the time is right.
For details on using the API, see our API documentation: https://www.yireo.com/tutorials/joomla/joomla-programming/1701-emailscheduler-api-for-joomla-developers