You may need to get the list of users who mailbox has reached the Quota limits. This information may help you to understand for capacity planning and understanding the datagrowth of the mailbox

Below poweshell helps you the list of mailbox whos which has reached Warning limits and ProhibitSend limits.

Get-MailboxStatistics -Server <Servername>| where{($_.StorageLimitStatus -contains “IssueWarning”) -or ($_.StorageLimitStatus -contains “ProhibitSend”)}