I have experience testing from a business perspective but I'm currently working in an system which is a small part of a bigger system, so I don't have a clear vision of the entire business value flow.
From a black box perspective its just a service with one input and one output:
Input: a cronjob starts the service and it will query a DB for specific cases where it should send an email;
Output: the system sends a reminder email and write in the DB that the email was sent.
The developers write unitary tests, integration tests and system test. Its pretty well covered by tests from a white box perspective.
From a black box perspective, which kind of test can I put in place?