There are two separate quotas for a Microsoft 365 mailbox. One is for the size of the mailbox. The other lesser-known limit is on the recoverable items database.

It is the recoverable items that frequently catch people off-guard. This quota isn’t shown in the admin portal dashboards. Furthermore, the limit is usually only an issue if you have a mailbox on legal hold.

These two things combine to make a frustrating situation. A user’s mailbox can stop due to a quota violation when it isn’t shown as being full on the portal. Here is a link to Microsoft’s documentation on the subject if you would like to know more.

Import-Module ExchangeOnlineManagement
Connect-ExchangeOnline
$Report = @()
$UserMailboxes = (Get-ExoMailbox -ResultSize unlimited -Filter {(RecipientType -eq ‘UserMailbox’)}).UserPrincipalName
Foreach ($Mailbox in $UserMailboxes){
    $MailboxTotalSize = [PSCustomObject]@{
        Name = $Mailbox
        Mailbox = Get-ExoMailboxStatistics -Identity $Mailbox | Select-Object TotalItemSize -ExpandProperty TotalItemSize
        RecoverableItems = Get-ExoMailboxFolderStatistics -Identity $Mailbox -FolderScope RecoverableItems | Select-Object Name,FolderAndSubfolderSize -ExpandProperty FolderAndSubfolderSize |Where-Object {$_.Name -like “*Recoverable*”}
    }
    $Report += $MailboxTotalSize
}
$Report |Export-Csv -Path
“C\Temp\MailboxTrueSizeReport.csv” -NoTypeInformation

I use the report to help prevent quota lockouts at my employer. You will need to run the script from a machine that has the Exchange Online Management Powershell module installed. Executing the script also requires an account with Exchange Online mailbox administrator permissions.

The output is an easy-to-read three-column report. The headers are Name (the user’s UPN), Mailbox, and RecoverableItems. The script takes around one hour per thousand mailboxes on average. This time can vary wildly,  the point is that it’s slow. Don’t let the computer running it go to sleep.