Why did I receive the notification email "backup of the above mailbox can continue if you free up some space or increase the quota on the mail-server side?"

This notification is sent by Dropsuite to alert you that your space on the Microsoft side of your mailbox has reached capacity, therefore backups will stop. To alleviate this message, please log in to the M365 portal and increase the storage space for the user or request the user delete some items in their mailbox to make room for new items.