As a design, the Email notification will get triggered only for Recurring Sync jobs [Real-time Sync(Scheduled)] sync failed with error. Not for the Real-Time Sync fails
For below operation:
Real-Time Sync - Erron email won't trigger when the sync fails to happen.
Real-time Sync(Scheduled) - Email will get it triggered and send it Customer when the Sync error happens.
Below Article FYI, This setting is available only to Recurring Sync jobs. - https://knowledgecenter.zuora.com/CPQ/E_Zuora_360/S_Synchronize_Data_from_Z-Billing
If you have any question, Please create a zuora support case
