Hi DTIOffice,
Currently, it is not available to send Exchange online calendar Appointments to SharePoint online by emailing.
There is a workaround that you can add the SharePoint calendar to user’s Outlook client. After that, user can add Appointments via Outlook client and it can be synchronized to SharePoint automatically. The main steps are as below.
1. Open the list that the calendar located in SharePoint online.
2. Click Connect to Outlook menu under Calendar Tools navigation.
3. Connect to Outlook client by following the steps of wizard.
Thanks,
Maggie Li