See:

https://theitbros.com/mailbox-size-and-quota-exchange-server/

Global Settings:  You can configure mailbox quotas from the Exchange admin center (EAC) web interface. To do this, go to the section Servers > Databases > select the required database > Properties > Limits.  Set the necessary quotas on the Limits tab.

 

 

Customize Per Mailbox:  Mailbox quota settings can also be changed from EAC (Recipients > Mailboxes > Properties > Mailbox usage > More Options > Customize the quota settings for this mailbox).

 

Example:  Change to “unlimited” instead of 2GB default maximum.

Issue a warning at (GB)  9.9

Prohibit Send at (GB)  “unlimited”

Prohibit send and receive at (GB) “unlimited”

Click “Save”