Hi @Muhammad Hussain , agree with the replies above, the mailbox quota defines the maximum value of sending/receiving/warning for the mailbox, it doesn't reserve the space for the mailbox.
Something more to clarify: from the database level, while adding data to Exchange mailboxes, the size of the Exchange database increases naturally. But, when it comes to deleting data, the size of the Exchange database may not reduce necessarily – it may remain the same.
For example, let’s say that you remove 4GB of data from the 30GB Exchange database, so the expected size of the database should be 26GB. But, after removing the data, you will notice that the size remains the same, which is 30 GB. However, there would still be 4 GB space available to add new data. This ‘Available new mailbox size’ is commonly known as Whitespace.
For the point above, referring to this link to get more information: How To Check Database White Space In Exchange
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