This article outlines how to force empty the deleted items / recoverable items dumpster in a users mailbox.
To view how much space is consumed by all users dumpster files run this command:
Get-MailBox | Get-MailboxStatistics | select DisplayName,TotalDeletedItemSize
To view an individuals TotalDeletedItemSize, run this command:
Get-MailboxStatistics <alias> | select DisplayName,TotalDeletedItemSize
To view how much white space / available space is in your database run this command:
Get-MailboxDatabase -Status | Sort-Object DatabaseSize -Descending | Format-Table Name, DatabaseSize, AvailableNewMailboxSpace
The idea now is to forcefully delete the TotalDeletedItemSize to increase the AvailableNewMailboxSpace in your mailbox databases.
For this we will use the Search-Mailbox command with the -SearchDumpsterOnly-DeleteContent switch.
This will delete all dumpster items from the users mailbox, which means the user will not be able to retrieve these items form Recover Deleted Items in Outlook
Search-mailbox -identity <alias> -SearchDumpsterOnly -DeleteContent