CRM synchronizes multiple attributes from crm to your outlook contacts and backwards. This includes the birthday of a contact. Based on this, you will get multiple notifications in your outlook, if you have many contacts.
While trying to enable and test the mailbox of a user, I got the following error message:
Appointments, contacts, and tasks can't be synchronized for the mailbox *** because the mailbox user doesn't have sufficient permissions on this mailbox.
Email Server Error Code: Crm.80048306.Not enough privilege to access the Microsoft Dynamics 365 object or perform the requested operation.