If an email to a customer bounces back, or the customer marks an email as spam, the system stops sending to that address to protect the delivery reputation of your other emails. The address is added to a suppression list.
When an address is added to the suppression list, the system sends a message to the school address (in Admin > Basic settings) so you're informed of any communication problems before they become a problem.
If a customer reports they have stopped receiving emails, first check the address on their account is correct. If the address is right and the problem has been resolved (for example, a full mailbox has been cleared), office staff can remove the address from the suppression list from the People area, and the system will start sending to it again.
Note: Only remove an address from the suppression list once you are confident the underlying problem is fixed, otherwise repeated bounces can harm delivery of your other emails and other flying schools using the system.