Exchange Mailbox storage best practices    

• Never store more than 10,000 items in one folder.
• Consider removing items from your Spam and Deleted Items folders.
• Do not exceed more than three levels of folders. For example:
   

• Best-practice folder level 1

  • Best-practice folder level 2

    • Best-practice folder level 3

      • Not a best-practice folder

Microsoft Exchange 2016 mailboxes can be up 50GB maximum, Exchange 2019 mailboxes can be made to go bigger but Microsoft does not recommend it.